

<!-- DFS ActiveDisclosure(SM) XBRL Linkbase Document - http://www.dfsco.com/financial-services/ -->
<!-- Creation Date      : 2018-05-14T19:46:10.4464906+00:00 -->
<!-- Version            : 2.9.6605.1333 -->
<!-- Package ID         : d58f19d9e0b34a68a68c36b94f7de083 -->
<!-- Copyright (c) 2018 Donnelley Financial, LLC. All Rights Reserved. -->
<linkbase xmlns="http://www.xbrl.org/2003/linkbase" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" arcroleURI="http://xbrl.org/int/dim/arcrole/all" />
  <arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" />
  <arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" />
  <arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" />
  <arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" />
  <roleRef xlink:type="simple" xlink:href="arct-20171231.xsd#TemplateLink" roleURI="http://www.alcobra-pharma.com/20171231/role/TemplateLink" />
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/StatementCONSOLIDATEDBALANCESHEETS" xlink:href="arct-20171231.xsd#Role_StatementCONSOLIDATEDBALANCESHEETS" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/StatementCONSOLIDATEDBALANCESHEETS">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:type="locator" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquity" xlink:type="locator" xlink:label="us-gaap_StockholdersEquity" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:type="locator" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:type="locator" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AdditionalPaidInCapital" xlink:type="locator" xlink:label="us-gaap_AdditionalPaidInCapital" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockValue" xlink:type="locator" xlink:label="us-gaap_CommonStockValue" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PreferredStockValue" xlink:type="locator" xlink:label="us-gaap_PreferredStockValue" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquityAbstract" xlink:type="locator" xlink:label="us-gaap_StockholdersEquityAbstract" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommitmentsAndContingencies" xlink:type="locator" xlink:label="us-gaap_CommitmentsAndContingencies" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_Liabilities" xlink:type="locator" xlink:label="us-gaap_Liabilities" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherLiabilities" xlink:type="locator" xlink:label="us-gaap_OtherLiabilities" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DeferredRevenueNoncurrent" xlink:type="locator" xlink:label="us-gaap_DeferredRevenueNoncurrent" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LiabilitiesCurrent" xlink:type="locator" xlink:label="us-gaap_LiabilitiesCurrent" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConvertibleDebtCurrent" xlink:type="locator" xlink:label="us-gaap_ConvertibleDebtCurrent" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DeferredRevenueCurrent" xlink:type="locator" xlink:label="us-gaap_DeferredRevenueCurrent" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccruedLiabilitiesCurrent" xlink:type="locator" xlink:label="us-gaap_AccruedLiabilitiesCurrent" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccountsPayableCurrent" xlink:type="locator" xlink:label="us-gaap_AccountsPayableCurrent" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LiabilitiesCurrentAbstract" xlink:type="locator" xlink:label="us-gaap_LiabilitiesCurrentAbstract" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:type="locator" xlink:label="us-gaap_LiabilitiesAndStockholdersEquityAbstract" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_Assets" xlink:type="locator" xlink:label="us-gaap_Assets" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherAssetsNoncurrent" xlink:type="locator" xlink:label="us-gaap_OtherAssetsNoncurrent" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AssetsCurrent" xlink:type="locator" xlink:label="us-gaap_AssetsCurrent" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill" />
    <loc xlink:href="arct-20171231.xsd#arct_PrepaidExpensesAndOtherReceivables" xlink:type="locator" xlink:label="arct_PrepaidExpensesAndOtherReceivables" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccountsReceivableNetCurrent" xlink:type="locator" xlink:label="us-gaap_AccountsReceivableNetCurrent" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShortTermInvestments" xlink:type="locator" xlink:label="us-gaap_ShortTermInvestments" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue" xlink:type="locator" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:type="locator" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AssetsCurrentAbstract" xlink:type="locator" xlink:label="us-gaap_AssetsCurrentAbstract" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AssetsAbstract" xlink:type="locator" xlink:label="us-gaap_AssetsAbstract" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SeriesAPreferredStockMember" xlink:type="locator" xlink:label="us-gaap_SeriesAPreferredStockMember" />
    <loc xlink:href="arct-20171231.xsd#arct_SeriesSeedPreferredStockMember" xlink:type="locator" xlink:label="arct_SeriesSeedPreferredStockMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:type="locator" xlink:label="us-gaap_ClassOfStockDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:type="locator" xlink:label="us-gaap_ClassOfStockDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:type="locator" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementTable" xlink:type="locator" xlink:label="us-gaap_StatementTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementLineItems" xlink:type="locator" xlink:label="us-gaap_StatementLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" order="10260.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementClassOfStockAxis" order="10290.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain_2" order="10300.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" order="10320.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="arct_SeriesSeedPreferredStockMember" order="10340.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_SeriesAPreferredStockMember" order="10360.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AssetsAbstract" order="10390.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_AssetsCurrentAbstract" order="10470.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_CashAndCashEquivalentsAtCarryingValue" order="10550.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue" order="10590.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_ShortTermInvestments" order="10630.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_AccountsReceivableNetCurrent" order="10670.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="arct_PrepaidExpensesAndOtherReceivables" order="10710.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill" order="10750.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_AssetsCurrent" order="10790.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" order="10830.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_OtherAssetsNoncurrent" order="10870.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_Assets" order="10910.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LiabilitiesAndStockholdersEquityAbstract" order="10950.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_LiabilitiesCurrentAbstract" order="11030.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_AccountsPayableCurrent" order="11110.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_AccruedLiabilitiesCurrent" order="11150.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_DeferredRevenueCurrent" order="11190.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_ConvertibleDebtCurrent" order="11230.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_LiabilitiesCurrent" order="11270.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_DeferredRevenueNoncurrent" order="11310.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_OtherLiabilities" order="11350.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_Liabilities" order="11390.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_CommitmentsAndContingencies" order="11430.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_StockholdersEquityAbstract" order="11470.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_PreferredStockValue" order="11550.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_CommonStockValue" order="11630.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_AdditionalPaidInCapital" order="11670.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" order="11710.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_RetainedEarningsAccumulatedDeficit" order="11750.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_StockholdersEquity" order="11790.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_LiabilitiesAndStockholdersEquity" order="11830.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/StatementCONSOLIDATEDBALANCESHEETSParenthetical" xlink:href="arct-20171231.xsd#Role_StatementCONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/StatementCONSOLIDATEDBALANCESHEETSParenthetical">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockSharesOutstanding" xlink:type="locator" xlink:label="us-gaap_CommonStockSharesOutstanding" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockSharesIssued" xlink:type="locator" xlink:label="us-gaap_CommonStockSharesIssued" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TreasuryStockCommonShares" xlink:type="locator" xlink:label="us-gaap_TreasuryStockCommonShares" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:type="locator" xlink:label="us-gaap_CommonStockSharesAuthorized" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:type="locator" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:type="locator" xlink:label="us-gaap_PreferredStockSharesOutstanding" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PreferredStockSharesIssued" xlink:type="locator" xlink:label="us-gaap_PreferredStockSharesIssued" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:type="locator" xlink:label="us-gaap_PreferredStockSharesAuthorized" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:type="locator" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SeriesAPreferredStockMember" xlink:type="locator" xlink:label="us-gaap_SeriesAPreferredStockMember" />
    <loc xlink:href="arct-20171231.xsd#arct_SeriesSeedPreferredStockMember" xlink:type="locator" xlink:label="arct_SeriesSeedPreferredStockMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:type="locator" xlink:label="us-gaap_ClassOfStockDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:type="locator" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementTable" xlink:type="locator" xlink:label="us-gaap_StatementTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementLineItems" xlink:type="locator" xlink:label="us-gaap_StatementLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementClassOfStockAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="arct_SeriesSeedPreferredStockMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_SeriesAPreferredStockMember" order="10170.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockParOrStatedValuePerShare" order="10260.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesAuthorized" order="10300.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesIssued" order="10340.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesOutstanding" order="10380.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockParOrStatedValuePerShare" order="10580.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesAuthorized" order="10620.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TreasuryStockCommonShares" order="10660.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesIssued" order="10700.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesOutstanding" order="10740.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/StatementSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYDEFICIT" xlink:href="arct-20171231.xsd#Role_StatementSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYDEFICIT" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/StatementSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYDEFICIT">
    <loc xlink:href="arct-20171231.xsd#arct_StockIssuedDuringPeriodSharesMergerNetOfRefundForFractionalShares" xlink:type="locator" xlink:label="arct_StockIssuedDuringPeriodSharesMergerNetOfRefundForFractionalShares" />
    <loc xlink:href="arct-20171231.xsd#arct_StockIssuedDuringPeriodValueMergerNetOfRefundForFractionalShares" xlink:type="locator" xlink:label="arct_StockIssuedDuringPeriodValueMergerNetOfRefundForFractionalShares" />
    <loc xlink:href="arct-20171231.xsd#arct_AdjustmentsToAdditionalPaidInCapitalBeneficialConversionExpenseFromNotes" xlink:type="locator" xlink:label="arct_AdjustmentsToAdditionalPaidInCapitalBeneficialConversionExpenseFromNotes" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" xlink:type="locator" xlink:label="us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities" xlink:type="locator" xlink:label="us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities" />
    <loc xlink:href="arct-20171231.xsd#arct_StockIssuedDuringPeriodSharesConversionOfDebtSecurities" xlink:type="locator" xlink:label="arct_StockIssuedDuringPeriodSharesConversionOfDebtSecurities" />
    <loc xlink:href="arct-20171231.xsd#arct_StockIssuedDuringPeriodValueConversionOfDebtSecurities" xlink:type="locator" xlink:label="arct_StockIssuedDuringPeriodValueConversionOfDebtSecurities" />
    <loc xlink:href="arct-20171231.xsd#arct_StockIssuedDuringPeriodSharesConversionOfDebtSecuritiesIncludingDebtConversionExpense" xlink:type="locator" xlink:label="arct_StockIssuedDuringPeriodSharesConversionOfDebtSecuritiesIncludingDebtConversionExpense" />
    <loc xlink:href="arct-20171231.xsd#arct_StockIssuedDuringPeriodValueConversionOfDebtSecuritiesIncludingDebtConversionExpense" xlink:type="locator" xlink:label="arct_StockIssuedDuringPeriodValueConversionOfDebtSecuritiesIncludingDebtConversionExpense" />
    <loc xlink:href="arct-20171231.xsd#arct_StockIssuedDuringPeriodSharesWarrantsExercised" xlink:type="locator" xlink:label="arct_StockIssuedDuringPeriodSharesWarrantsExercised" />
    <loc xlink:href="arct-20171231.xsd#arct_StockIssuedDuringPeriodValueWarrantsExercised" xlink:type="locator" xlink:label="arct_StockIssuedDuringPeriodValueWarrantsExercised" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:type="locator" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:type="locator" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:type="locator" xlink:label="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" xlink:type="locator" xlink:label="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NetIncomeLoss" xlink:type="locator" xlink:label="us-gaap_NetIncomeLoss" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharesOutstanding" xlink:type="locator" xlink:label="us-gaap_SharesOutstanding" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquity" xlink:type="locator" xlink:label="us-gaap_StockholdersEquity" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SeriesAPreferredStockMember" xlink:type="locator" xlink:label="us-gaap_SeriesAPreferredStockMember" />
    <loc xlink:href="arct-20171231.xsd#arct_SeriesSeedPreferredStockMember" xlink:type="locator" xlink:label="arct_SeriesSeedPreferredStockMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:type="locator" xlink:label="us-gaap_ClassOfStockDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:type="locator" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:type="locator" xlink:label="us-gaap_RetainedEarningsMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:type="locator" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:type="locator" xlink:label="us-gaap_AdditionalPaidInCapitalMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockMember" xlink:type="locator" xlink:label="us-gaap_CommonStockMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EquityComponentDomain" xlink:type="locator" xlink:label="us-gaap_EquityComponentDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EquityComponentDomain" xlink:type="locator" xlink:label="us-gaap_EquityComponentDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:type="locator" xlink:label="us-gaap_StatementEquityComponentsAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementTable" xlink:type="locator" xlink:label="us-gaap_StatementTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementLineItems" xlink:type="locator" xlink:label="us-gaap_StatementLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementEquityComponentsAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain_2" order="10050.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_CommonStockMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_AdditionalPaidInCapitalMember" order="10130.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeMember" order="10150.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_RetainedEarningsMember" order="10170.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementClassOfStockAxis" order="10180.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" order="10210.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="arct_SeriesSeedPreferredStockMember" order="10230.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_SeriesAPreferredStockMember" order="10270.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockholdersEquity" order="10320.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharesOutstanding" order="10340.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetIncomeLoss" order="10540.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" order="10760.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" order="10980.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" order="11080.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" order="11200.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="arct_StockIssuedDuringPeriodValueWarrantsExercised" order="11420.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="arct_StockIssuedDuringPeriodSharesWarrantsExercised" order="11640.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="arct_StockIssuedDuringPeriodValueConversionOfDebtSecuritiesIncludingDebtConversionExpense" order="11860.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="arct_StockIssuedDuringPeriodSharesConversionOfDebtSecuritiesIncludingDebtConversionExpense" order="12080.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="arct_StockIssuedDuringPeriodValueConversionOfDebtSecurities" order="12300.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="arct_StockIssuedDuringPeriodSharesConversionOfDebtSecurities" order="12520.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities" order="12740.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" order="12960.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="arct_AdjustmentsToAdditionalPaidInCapitalBeneficialConversionExpenseFromNotes" order="13180.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="arct_StockIssuedDuringPeriodValueMergerNetOfRefundForFractionalShares" order="13400.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="arct_StockIssuedDuringPeriodSharesMergerNetOfRefundForFractionalShares" order="13620.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureOrganizationDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureOrganizationDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureOrganizationDetailsTextual">
    <loc xlink:href="arct-20171231.xsd#arct_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalentsAndShortTermInvestments" xlink:type="locator" xlink:label="arct_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalentsAndShortTermInvestments" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:type="locator" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" xlink:type="locator" xlink:label="us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquityReverseStockSplit" xlink:type="locator" xlink:label="us-gaap_StockholdersEquityReverseStockSplit" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:type="locator" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesAcquisitions" xlink:type="locator" xlink:label="us-gaap_StockIssuedDuringPeriodSharesAcquisitions" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessAcquisitionEffectiveDateOfAcquisition1" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionEffectiveDateOfAcquisition1" />
    <loc xlink:href="arct-20171231.xsd#arct_AlcobraLimitedMember" xlink:type="locator" xlink:label="arct_AlcobraLimitedMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_OrganizationAndSignificantAccountingPoliciesTable" xlink:type="locator" xlink:label="arct_OrganizationAndSignificantAccountingPoliciesTable" />
    <loc xlink:href="arct-20171231.xsd#arct_OrganizationAndSignificantAccountingPoliciesLineItems" xlink:type="locator" xlink:label="arct_OrganizationAndSignificantAccountingPoliciesLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="arct_OrganizationAndSignificantAccountingPoliciesLineItems" xlink:to="arct_OrganizationAndSignificantAccountingPoliciesTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_OrganizationAndSignificantAccountingPoliciesTable" xlink:to="us-gaap_BusinessAcquisitionAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain_2" order="10050.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="arct_AlcobraLimitedMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_OrganizationAndSignificantAccountingPoliciesLineItems" xlink:to="us-gaap_BusinessAcquisitionEffectiveDateOfAcquisition1" order="10140.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_OrganizationAndSignificantAccountingPoliciesLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesAcquisitions" order="10160.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_OrganizationAndSignificantAccountingPoliciesLineItems" xlink:to="us-gaap_CommonStockParOrStatedValuePerShare" order="10180.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_OrganizationAndSignificantAccountingPoliciesLineItems" xlink:to="us-gaap_StockholdersEquityReverseStockSplit" order="10200.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_OrganizationAndSignificantAccountingPoliciesLineItems" xlink:to="us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" order="10220.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_OrganizationAndSignificantAccountingPoliciesLineItems" xlink:to="us-gaap_RetainedEarningsAccumulatedDeficit" order="10270.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_OrganizationAndSignificantAccountingPoliciesLineItems" xlink:to="arct_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalentsAndShortTermInvestments" order="10290.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureSummaryOfSignificantAccountingPoliciesDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesDetailsTextual">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PaymentsOfDividendsCommonStock" xlink:type="locator" xlink:label="us-gaap_PaymentsOfDividendsCommonStock" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DividendsCommonStock" xlink:type="locator" xlink:label="us-gaap_DividendsCommonStock" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TreasuryStockCommonShares" xlink:type="locator" xlink:label="us-gaap_TreasuryStockCommonShares" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:type="locator" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" />
    <loc xlink:href="arct-20171231.xsd#arct_MultipleUsedToEstimateValueOfShares" xlink:type="locator" xlink:label="arct_MultipleUsedToEstimateValueOfShares" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRights" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRights" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:type="locator" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" xlink:type="locator" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" />
    <loc xlink:href="arct-20171231.xsd#arct_DecreaseInResearchAndDevelopmentGrantsReceived" xlink:type="locator" xlink:label="arct_DecreaseInResearchAndDevelopmentGrantsReceived" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DeferredRevenue" xlink:type="locator" xlink:label="us-gaap_DeferredRevenue" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_GoodwillImpairmentLoss" xlink:type="locator" xlink:label="us-gaap_GoodwillImpairmentLoss" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentUsefulLife" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConcentrationRiskPercentage1" xlink:type="locator" xlink:label="us-gaap_ConcentrationRiskPercentage1" />
    <loc xlink:href="arct-20171231.xsd#arct_ConcentrationRiskNumberOfCustomer" xlink:type="locator" xlink:label="arct_ConcentrationRiskNumberOfCustomer" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivable" xlink:type="locator" xlink:label="us-gaap_AllowanceForDoubtfulAccountsReceivable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RestrictedCash" xlink:type="locator" xlink:label="us-gaap_RestrictedCash" />
    <loc xlink:href="arct-20171231.xsd#arct_LeaseExpirationYear" xlink:type="locator" xlink:label="arct_LeaseExpirationYear" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RestrictedCashNoncurrent" xlink:type="locator" xlink:label="us-gaap_RestrictedCashNoncurrent" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RestrictedCashCurrent" xlink:type="locator" xlink:label="us-gaap_RestrictedCashCurrent" />
    <loc xlink:href="arct-20171231.xsd#arct_StockOptionsAndRestrictedStockMember" xlink:type="locator" xlink:label="arct_StockOptionsAndRestrictedStockMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:type="locator" xlink:label="us-gaap_AntidilutiveSecuritiesNameDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:type="locator" xlink:label="us-gaap_AntidilutiveSecuritiesNameDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:type="locator" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationAwardTrancheOneMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_VestingDomain" xlink:type="locator" xlink:label="us-gaap_VestingDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_VestingDomain" xlink:type="locator" xlink:label="us-gaap_VestingDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_VestingAxis" xlink:type="locator" xlink:label="us-gaap_VestingAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_ResearchCollaborationAndExclusiveLicenseAgreementMember" xlink:type="locator" xlink:label="arct_ResearchCollaborationAndExclusiveLicenseAgreementMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:type="locator" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:type="locator" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:type="locator" xlink:label="us-gaap_TypeOfArrangementAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_OtherCollaborationMember" xlink:type="locator" xlink:label="arct_OtherCollaborationMember" />
    <loc xlink:href="arct-20171231.xsd#arct_CollaborationPartnerCMember" xlink:type="locator" xlink:label="arct_CollaborationPartnerCMember" />
    <loc xlink:href="arct-20171231.xsd#arct_CollaborationPartnerBMember" xlink:type="locator" xlink:label="arct_CollaborationPartnerBMember" />
    <loc xlink:href="arct-20171231.xsd#arct_CollaborationPartnerAMember" xlink:type="locator" xlink:label="arct_CollaborationPartnerAMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="locator" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="locator" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CounterpartyNameAxis" xlink:type="locator" xlink:label="us-gaap_CounterpartyNameAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MaximumMember" xlink:type="locator" xlink:label="us-gaap_MaximumMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MinimumMember" xlink:type="locator" xlink:label="us-gaap_MinimumMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeMember" xlink:type="locator" xlink:label="us-gaap_RangeMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeMember" xlink:type="locator" xlink:label="us-gaap_RangeMember_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeAxis" xlink:type="locator" xlink:label="us-gaap_RangeAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SalesRevenueNetMember" xlink:type="locator" xlink:label="us-gaap_SalesRevenueNetMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccountsReceivableMember" xlink:type="locator" xlink:label="us-gaap_AccountsReceivableMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:type="locator" xlink:label="us-gaap_ConcentrationRiskBenchmarkDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:type="locator" xlink:label="us-gaap_ConcentrationRiskBenchmarkDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:type="locator" xlink:label="us-gaap_ConcentrationRiskByBenchmarkAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CreditConcentrationRiskMember" xlink:type="locator" xlink:label="us-gaap_CreditConcentrationRiskMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:type="locator" xlink:label="us-gaap_ConcentrationRiskTypeDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:type="locator" xlink:label="us-gaap_ConcentrationRiskTypeDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConcentrationRiskByTypeAxis" xlink:type="locator" xlink:label="us-gaap_ConcentrationRiskByTypeAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_ScheduleOfSummaryOfSignificantAccountingPoliciesTable" xlink:type="locator" xlink:label="arct_ScheduleOfSummaryOfSignificantAccountingPoliciesTable" />
    <loc xlink:href="arct-20171231.xsd#arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:type="locator" xlink:label="arct_SummaryOfSignificantAccountingPolicyLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="arct_ScheduleOfSummaryOfSignificantAccountingPoliciesTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_ScheduleOfSummaryOfSignificantAccountingPoliciesTable" xlink:to="us-gaap_ConcentrationRiskByTypeAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ConcentrationRiskByTypeAxis" xlink:to="us-gaap_ConcentrationRiskTypeDomain_2" order="10050.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ConcentrationRiskByTypeAxis" xlink:to="us-gaap_ConcentrationRiskTypeDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ConcentrationRiskTypeDomain" xlink:to="us-gaap_CreditConcentrationRiskMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_ScheduleOfSummaryOfSignificantAccountingPoliciesTable" xlink:to="us-gaap_ConcentrationRiskByBenchmarkAxis" order="10160.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:to="us-gaap_ConcentrationRiskBenchmarkDomain_2" order="10170.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:to="us-gaap_ConcentrationRiskBenchmarkDomain" order="10190.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ConcentrationRiskBenchmarkDomain" xlink:to="us-gaap_AccountsReceivableMember" order="10210.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ConcentrationRiskBenchmarkDomain" xlink:to="us-gaap_SalesRevenueNetMember" order="10250.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_ScheduleOfSummaryOfSignificantAccountingPoliciesTable" xlink:to="us-gaap_RangeAxis" order="10280.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember_2" order="10290.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" order="10310.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MinimumMember" order="10330.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MaximumMember" order="10350.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_ScheduleOfSummaryOfSignificantAccountingPoliciesTable" xlink:to="us-gaap_CounterpartyNameAxis" order="10360.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain_2" order="10370.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain" order="10390.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="arct_CollaborationPartnerAMember" order="10410.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="arct_CollaborationPartnerBMember" order="10430.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="arct_CollaborationPartnerCMember" order="10450.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="arct_OtherCollaborationMember" order="10470.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_ScheduleOfSummaryOfSignificantAccountingPoliciesTable" xlink:to="us-gaap_TypeOfArrangementAxis" order="10480.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember_2" order="10490.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember" order="10510.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="arct_ResearchCollaborationAndExclusiveLicenseAgreementMember" order="10530.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_ScheduleOfSummaryOfSignificantAccountingPoliciesTable" xlink:to="us-gaap_VestingAxis" order="10600.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_VestingAxis" xlink:to="us-gaap_VestingDomain_2" order="10610.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_VestingAxis" xlink:to="us-gaap_VestingDomain" order="10630.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="us-gaap_ShareBasedCompensationAwardTrancheOneMember" order="10650.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_ScheduleOfSummaryOfSignificantAccountingPoliciesTable" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" order="10740.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:to="us-gaap_AntidilutiveSecuritiesNameDomain_2" order="10750.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:to="us-gaap_AntidilutiveSecuritiesNameDomain" order="10770.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AntidilutiveSecuritiesNameDomain" xlink:to="arct_StockOptionsAndRestrictedStockMember" order="10790.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_RestrictedCashCurrent" order="10820.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_RestrictedCashNoncurrent" order="10840.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="arct_LeaseExpirationYear" order="10860.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_RestrictedCash" order="10880.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_AllowanceForDoubtfulAccountsReceivable" order="10930.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="arct_ConcentrationRiskNumberOfCustomer" order="11000.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_ConcentrationRiskPercentage1" order="11060.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentUsefulLife" order="11270.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_GoodwillImpairmentLoss" order="11390.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_DeferredRevenue" order="11480.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="arct_DecreaseInResearchAndDevelopmentGrantsReceived" order="11670.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" order="11740.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" order="11760.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" order="11780.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRights" order="11800.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="arct_MultipleUsedToEstimateValueOfShares" order="11850.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" order="11920.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_TreasuryStockCommonShares" order="12070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_DividendsCommonStock" order="12160.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_SummaryOfSignificantAccountingPolicyLineItems" xlink:to="us-gaap_PaymentsOfDividendsCommonStock" order="12220.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureStrategicAlliancesAndCollaborationAgreementsSummaryOfRevenuesUnderStrategicAlliancesAndCollaborationsDetails" xlink:href="arct-20171231.xsd#Role_DisclosureStrategicAlliancesAndCollaborationAgreementsSummaryOfRevenuesUnderStrategicAlliancesAndCollaborationsDetails" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureStrategicAlliancesAndCollaborationAgreementsSummaryOfRevenuesUnderStrategicAlliancesAndCollaborationsDetails">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_Revenues" xlink:type="locator" xlink:label="us-gaap_Revenues" />
    <loc xlink:href="arct-20171231.xsd#arct_OtherCollaborationMember" xlink:type="locator" xlink:label="arct_OtherCollaborationMember" />
    <loc xlink:href="arct-20171231.xsd#arct_CollaborationPartnerCMember" xlink:type="locator" xlink:label="arct_CollaborationPartnerCMember" />
    <loc xlink:href="arct-20171231.xsd#arct_CollaborationPartnerBMember" xlink:type="locator" xlink:label="arct_CollaborationPartnerBMember" />
    <loc xlink:href="arct-20171231.xsd#arct_CollaborationPartnerAMember" xlink:type="locator" xlink:label="arct_CollaborationPartnerAMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="locator" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CounterpartyNameAxis" xlink:type="locator" xlink:label="us-gaap_CounterpartyNameAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_ResearchCollaborationAndExclusiveLicenseAgreementMember" xlink:type="locator" xlink:label="arct_ResearchCollaborationAndExclusiveLicenseAgreementMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:type="locator" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:type="locator" xlink:label="us-gaap_TypeOfArrangementAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:type="locator" xlink:label="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:to="us-gaap_TypeOfArrangementAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="arct_ResearchCollaborationAndExclusiveLicenseAgreementMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:to="us-gaap_CounterpartyNameAxis" order="10180.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain" order="10210.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="arct_CollaborationPartnerAMember" order="10230.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="arct_CollaborationPartnerBMember" order="10250.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="arct_CollaborationPartnerCMember" order="10270.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="arct_OtherCollaborationMember" order="10290.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_Revenues" order="10320.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureStrategicAlliancesAndCollaborationAgreementsDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureStrategicAlliancesAndCollaborationAgreementsDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureStrategicAlliancesAndCollaborationAgreementsDetailsTextual">
    <loc xlink:href="arct-20171231.xsd#arct_ResearchFundingPaymentsReceived" xlink:type="locator" xlink:label="arct_ResearchFundingPaymentsReceived" />
    <loc xlink:href="arct-20171231.xsd#arct_UpfrontFeeReceived" xlink:type="locator" xlink:label="arct_UpfrontFeeReceived" />
    <loc xlink:href="arct-20171231.xsd#arct_RevenueRecognizedAsOneTimePayment" xlink:type="locator" xlink:label="arct_RevenueRecognizedAsOneTimePayment" />
    <loc xlink:href="arct-20171231.xsd#arct_RoyaltyPaymentPercentage" xlink:type="locator" xlink:label="arct_RoyaltyPaymentPercentage" />
    <loc xlink:href="arct-20171231.xsd#arct_OptionExerciseRevenueRangePerTarget" xlink:type="locator" xlink:label="arct_OptionExerciseRevenueRangePerTarget" />
    <loc xlink:href="arct-20171231.xsd#arct_RevenueRecognitionPotentialMilestoneRevenueRecognized" xlink:type="locator" xlink:label="arct_RevenueRecognitionPotentialMilestoneRevenueRecognized" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ReimbursementRevenue" xlink:type="locator" xlink:label="us-gaap_ReimbursementRevenue" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_Revenues" xlink:type="locator" xlink:label="us-gaap_Revenues" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RevenueRecognitionMilestoneMethodMilestone" xlink:type="locator" xlink:label="us-gaap_RevenueRecognitionMilestoneMethodMilestone" />
    <loc xlink:href="arct-20171231.xsd#arct_ResearchCollaborationLicenseAdditionalExtensionPeriod" xlink:type="locator" xlink:label="arct_ResearchCollaborationLicenseAdditionalExtensionPeriod" />
    <loc xlink:href="arct-20171231.xsd#arct_RevenueRecognitionResearchActivitiesPeriod" xlink:type="locator" xlink:label="arct_RevenueRecognitionResearchActivitiesPeriod" />
    <loc xlink:href="arct-20171231.xsd#arct_NumberOfCollaborationAgreements" xlink:type="locator" xlink:label="arct_NumberOfCollaborationAgreements" />
    <loc xlink:href="arct-20171231.xsd#arct_CollaborationPartnerCMember" xlink:type="locator" xlink:label="arct_CollaborationPartnerCMember" />
    <loc xlink:href="arct-20171231.xsd#arct_CollaborationPartnerBMember" xlink:type="locator" xlink:label="arct_CollaborationPartnerBMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MaximumMember" xlink:type="locator" xlink:label="us-gaap_MaximumMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MinimumMember" xlink:type="locator" xlink:label="us-gaap_MinimumMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeMember" xlink:type="locator" xlink:label="us-gaap_RangeMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeAxis" xlink:type="locator" xlink:label="us-gaap_RangeAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_ResearchCollaborationAndExclusiveLicenseAgreementMember" xlink:type="locator" xlink:label="arct_ResearchCollaborationAndExclusiveLicenseAgreementMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:type="locator" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:type="locator" xlink:label="us-gaap_TypeOfArrangementAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_CollaborationPartnerAMember" xlink:type="locator" xlink:label="arct_CollaborationPartnerAMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="locator" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CounterpartyNameAxis" xlink:type="locator" xlink:label="us-gaap_CounterpartyNameAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:type="locator" xlink:label="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:to="us-gaap_CounterpartyNameAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="arct_CollaborationPartnerAMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:to="us-gaap_TypeOfArrangementAxis" order="10100.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember" order="10130.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="arct_ResearchCollaborationAndExclusiveLicenseAgreementMember" order="10150.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:to="us-gaap_RangeAxis" order="10720.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" order="10750.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MinimumMember" order="10770.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MaximumMember" order="10790.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="arct_CollaborationPartnerBMember" order="10850.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="arct_CollaborationPartnerCMember" order="11490.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="arct_NumberOfCollaborationAgreements" order="11900.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="arct_RevenueRecognitionResearchActivitiesPeriod" order="11950.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="arct_ResearchCollaborationLicenseAdditionalExtensionPeriod" order="11970.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_RevenueRecognitionMilestoneMethodMilestone" order="11990.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_Revenues" order="12040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_ReimbursementRevenue" order="12100.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="arct_RevenueRecognitionPotentialMilestoneRevenueRecognized" order="12190.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="arct_OptionExerciseRevenueRangePerTarget" order="12210.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="arct_RoyaltyPaymentPercentage" order="12350.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="arct_RevenueRecognizedAsOneTimePayment" order="12400.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="arct_UpfrontFeeReceived" order="12670.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="arct_ResearchFundingPaymentsReceived" order="12690.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureShortTermInvestmentsDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureShortTermInvestmentsDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureShortTermInvestmentsDetailsTextual">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AvailableForSaleSecuritiesCurrent" xlink:type="locator" xlink:label="us-gaap_AvailableForSaleSecuritiesCurrent" />
    <loc xlink:href="arct-20171231.xsd#arct_ShortTermDepositWeightedAverageInterestRate" xlink:type="locator" xlink:label="arct_ShortTermDepositWeightedAverageInterestRate" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShortTermInvestments" xlink:type="locator" xlink:label="us-gaap_ShortTermInvestments" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BankTimeDepositsMember" xlink:type="locator" xlink:label="us-gaap_BankTimeDepositsMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:type="locator" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:type="locator" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CashAndCashEquivalentsAxis" xlink:type="locator" xlink:label="us-gaap_CashAndCashEquivalentsAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:type="locator" xlink:label="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:to="us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" xlink:to="us-gaap_CashAndCashEquivalentsAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_CashAndCashEquivalentsAxis" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_2" order="10050.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CashAndCashEquivalentsAxis" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:to="us-gaap_BankTimeDepositsMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:to="us-gaap_ShortTermInvestments" order="10120.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:to="arct_ShortTermDepositWeightedAverageInterestRate" order="10160.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesCurrent" order="10230.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureSummaryOfShortTermInvestmentsDetails" xlink:href="arct-20171231.xsd#Role_DisclosureSummaryOfShortTermInvestmentsDetails" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureSummaryOfShortTermInvestmentsDetails">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities" xlink:type="locator" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtSecurities" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:type="locator" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:type="locator" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis" xlink:type="locator" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CertificatesOfDepositMember" xlink:type="locator" xlink:label="us-gaap_CertificatesOfDepositMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_InvestmentTypeCategorizationMember" xlink:type="locator" xlink:label="us-gaap_InvestmentTypeCategorizationMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_InvestmentTypeCategorizationMember" xlink:type="locator" xlink:label="us-gaap_InvestmentTypeCategorizationMember_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_InvestmentTypeAxis" xlink:type="locator" xlink:label="us-gaap_InvestmentTypeAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:type="locator" xlink:label="us-gaap_CorporateDebtSecuritiesMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:type="locator" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:type="locator" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:type="locator" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:type="locator" xlink:label="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:to="us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" order="10040.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" xlink:to="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:to="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_2" order="10080.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:to="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" order="10100.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:to="us-gaap_CorporateDebtSecuritiesMember" order="10120.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" xlink:to="us-gaap_InvestmentTypeAxis" order="10130.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_InvestmentTypeAxis" xlink:to="us-gaap_InvestmentTypeCategorizationMember_2" order="10140.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_InvestmentTypeAxis" xlink:to="us-gaap_InvestmentTypeCategorizationMember" order="10160.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InvestmentTypeCategorizationMember" xlink:to="us-gaap_CertificatesOfDepositMember" order="10180.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:to="us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis" order="10210.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:to="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" order="10310.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:to="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" order="10410.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesDebtSecurities" order="10510.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureScheduleOfFinancialAssetsMeasuredAtFairValueOnRecurringBasisDetails" xlink:href="arct-20171231.xsd#Role_DisclosureScheduleOfFinancialAssetsMeasuredAtFairValueOnRecurringBasisDetails" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureScheduleOfFinancialAssetsMeasuredAtFairValueOnRecurringBasisDetails">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AssetsFairValueDisclosureRecurring" xlink:type="locator" xlink:label="us-gaap_AssetsFairValueDisclosureRecurring" />
    <loc xlink:href="arct-20171231.xsd#arct_CorporateDebtSecuritiesFairValueDisclosure" xlink:type="locator" xlink:label="arct_CorporateDebtSecuritiesFairValueDisclosure" />
    <loc xlink:href="arct-20171231.xsd#arct_CertificatesOfDepositFairValueDisclosure" xlink:type="locator" xlink:label="arct_CertificatesOfDepositFairValueDisclosure" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure" xlink:type="locator" xlink:label="us-gaap_CashAndCashEquivalentsFairValueDisclosure" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AssetsFairValueDisclosureAbstract" xlink:type="locator" xlink:label="us-gaap_AssetsFairValueDisclosureAbstract" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:type="locator" xlink:label="us-gaap_FairValueInputsLevel3Member" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:type="locator" xlink:label="us-gaap_FairValueInputsLevel2Member" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:type="locator" xlink:label="us-gaap_FairValueInputsLevel1Member" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="locator" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="locator" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="locator" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:type="locator" xlink:label="us-gaap_FairValueMeasurementsRecurringMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:type="locator" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:type="locator" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:type="locator" xlink:label="us-gaap_FairValueByMeasurementFrequencyAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:type="locator" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:type="locator" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" order="10040.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="us-gaap_FairValueByMeasurementFrequencyAxis" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:to="us-gaap_FairValueMeasurementFrequencyDomain_2" order="10080.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:to="us-gaap_FairValueMeasurementFrequencyDomain" order="10100.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementFrequencyDomain" xlink:to="us-gaap_FairValueMeasurementsRecurringMember" order="10120.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis" order="10190.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2" order="10200.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" order="10220.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel1Member" order="10240.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel2Member" order="10260.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel3Member" order="10280.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="us-gaap_AssetsFairValueDisclosureAbstract" order="10310.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsFairValueDisclosureAbstract" xlink:to="us-gaap_CashAndCashEquivalentsFairValueDisclosure" order="10470.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsFairValueDisclosureAbstract" xlink:to="arct_CertificatesOfDepositFairValueDisclosure" order="10550.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsFairValueDisclosureAbstract" xlink:to="arct_CorporateDebtSecuritiesFairValueDisclosure" order="10630.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsFairValueDisclosureAbstract" xlink:to="us-gaap_AssetsFairValueDisclosureRecurring" order="10710.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureFairValueMeasurementsDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureFairValueMeasurementsDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureFairValueMeasurementsDetailsTextual">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AssetsFairValueDisclosureRecurring" xlink:type="locator" xlink:label="us-gaap_AssetsFairValueDisclosureRecurring" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:type="locator" xlink:label="us-gaap_FairValueMeasurementsRecurringMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:type="locator" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:type="locator" xlink:label="us-gaap_FairValueByMeasurementFrequencyAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:type="locator" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:type="locator" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="us-gaap_FairValueByMeasurementFrequencyAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:to="us-gaap_FairValueMeasurementFrequencyDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementFrequencyDomain" xlink:to="us-gaap_FairValueMeasurementsRecurringMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="us-gaap_AssetsFairValueDisclosureRecurring" order="10120.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureReverseMergerWithAlcobraLtdDetails1" xlink:href="arct-20171231.xsd#Role_DisclosureReverseMergerWithAlcobraLtdDetails1" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureReverseMergerWithAlcobraLtdDetails1">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsMarketableSecurities" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsMarketableSecurities" />
    <loc xlink:href="arct-20171231.xsd#arct_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRestrictedCash" xlink:type="locator" xlink:label="arct_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRestrictedCash" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents" />
    <loc xlink:href="arct-20171231.xsd#arct_AlcobraLimitedMember" xlink:type="locator" xlink:label="arct_AlcobraLimitedMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" order="10090.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="us-gaap_BusinessAcquisitionAxis" order="10120.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" order="10150.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="arct_AlcobraLimitedMember" order="10170.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents" order="10200.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="arct_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRestrictedCash" order="10220.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsMarketableSecurities" order="10240.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets" order="10260.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment" order="10280.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill" order="10300.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets" order="10320.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable" order="10340.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet" order="10360.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureReverseMergerWithAlcobraLtdDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureReverseMergerWithAlcobraLtdDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureReverseMergerWithAlcobraLtdDetailsTextual">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EquityMethodInvestmentDescriptionOfPrincipalActivities" xlink:type="locator" xlink:label="us-gaap_EquityMethodInvestmentDescriptionOfPrincipalActivities" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage" xlink:type="locator" xlink:label="us-gaap_EquityMethodInvestmentOwnershipPercentage" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PaymentsForMergerRelatedCosts" xlink:type="locator" xlink:label="us-gaap_PaymentsForMergerRelatedCosts" />
    <loc xlink:href="arct-20171231.xsd#arct_BusinessCombinationChargeToAdditionalPaidInCapital" xlink:type="locator" xlink:label="arct_BusinessCombinationChargeToAdditionalPaidInCapital" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessAcquisitionSharePrice" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionSharePrice" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharesOutstanding" xlink:type="locator" xlink:label="us-gaap_SharesOutstanding" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferred1" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationConsiderationTransferred1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EquityMethodInvesteeNameDomain" xlink:type="locator" xlink:label="us-gaap_EquityMethodInvesteeNameDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EquityMethodInvesteeNameDomain" xlink:type="locator" xlink:label="us-gaap_EquityMethodInvesteeNameDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:type="locator" xlink:label="us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_AlcobraLimitedMember" xlink:type="locator" xlink:label="arct_AlcobraLimitedMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_AmiserviceMember" xlink:type="locator" xlink:label="arct_AmiserviceMember" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:type="locator" xlink:label="dei_EntityDomain" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:type="locator" xlink:label="dei_EntityDomain_2" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_LegalEntityAxis" xlink:type="locator" xlink:label="dei_LegalEntityAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="dei_LegalEntityAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain_2" order="10050.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_EntityDomain" xlink:to="arct_AmiserviceMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="us-gaap_BusinessAcquisitionAxis" order="10100.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" order="10130.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="arct_AlcobraLimitedMember" order="10150.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" order="10240.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="us-gaap_EquityMethodInvesteeNameDomain_2" order="10250.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="us-gaap_EquityMethodInvesteeNameDomain" order="10270.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityMethodInvesteeNameDomain" xlink:to="arct_AmiserviceMember" order="10290.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationConsiderationTransferred1" order="10320.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_SharesOutstanding" order="10340.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessAcquisitionSharePrice" order="10360.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="arct_BusinessCombinationChargeToAdditionalPaidInCapital" order="10380.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_PaymentsForMergerRelatedCosts" order="10400.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_EquityMethodInvestmentOwnershipPercentage" order="10420.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_EquityMethodInvestmentDescriptionOfPrincipalActivities" order="10470.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosurePropertyAndEquipmentNetDetails" xlink:href="arct-20171231.xsd#Role_DisclosurePropertyAndEquipmentNetDetails" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosurePropertyAndEquipmentNetDetails">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:type="locator" xlink:label="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentGross" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LeaseholdImprovementsMember" xlink:type="locator" xlink:label="us-gaap_LeaseholdImprovementsMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:type="locator" xlink:label="us-gaap_FurnitureAndFixturesMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ComputerEquipmentMember" xlink:type="locator" xlink:label="us-gaap_ComputerEquipmentMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EquipmentMember" xlink:type="locator" xlink:label="us-gaap_EquipmentMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" order="10030.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" order="10060.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain_2" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_EquipmentMember" order="10110.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ComputerEquipmentMember" order="10130.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_FurnitureAndFixturesMember" order="10150.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_LeaseholdImprovementsMember" order="10170.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentGross" order="10200.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" order="10400.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" order="10440.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureConvertiblePromissoryNotesDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureConvertiblePromissoryNotesDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureConvertiblePromissoryNotesDetailsTextual">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_InterestExpense" xlink:type="locator" xlink:label="us-gaap_InterestExpense" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentConvertibleConversionPrice1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtConversionConvertedInstrumentAmount1" xlink:type="locator" xlink:label="us-gaap_DebtConversionConvertedInstrumentAmount1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConvertibleNotesPayable" xlink:type="locator" xlink:label="us-gaap_ConvertibleNotesPayable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RepaymentsOfConvertibleDebt" xlink:type="locator" xlink:label="us-gaap_RepaymentsOfConvertibleDebt" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_InducedConversionOfConvertibleDebtExpense" xlink:type="locator" xlink:label="us-gaap_InducedConversionOfConvertibleDebtExpense" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtConversionConvertedInstrumentSharesIssued1" xlink:type="locator" xlink:label="us-gaap_DebtConversionConvertedInstrumentSharesIssued1" />
    <loc xlink:href="arct-20171231.xsd#arct_AccruedInterestOnPromissoryNote" xlink:type="locator" xlink:label="arct_AccruedInterestOnPromissoryNote" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtConversionOriginalDebtAmount1" xlink:type="locator" xlink:label="us-gaap_DebtConversionOriginalDebtAmount1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentInterestRateStatedPercentage" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtInstrumentMaturityDate" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentMaturityDate" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProceedsFromConvertibleDebt" xlink:type="locator" xlink:label="us-gaap_ProceedsFromConvertibleDebt" />
    <loc xlink:href="arct-20171231.xsd#arct_MinimumPercentageOfCapitalStockOwnershipOfInvestorsToWhichConvertibleNotesIssued" xlink:type="locator" xlink:label="arct_MinimumPercentageOfCapitalStockOwnershipOfInvestorsToWhichConvertibleNotesIssued" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SeriesAPreferredStockMember" xlink:type="locator" xlink:label="us-gaap_SeriesAPreferredStockMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:type="locator" xlink:label="us-gaap_ClassOfStockDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:type="locator" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_SecondConvertiblePromissoryNotesMember" xlink:type="locator" xlink:label="arct_SecondConvertiblePromissoryNotesMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentNameDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentNameDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConvertibleNotesPayableMember" xlink:type="locator" xlink:label="us-gaap_ConvertibleNotesPayableMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:type="locator" xlink:label="us-gaap_ShortTermDebtTypeDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:type="locator" xlink:label="us-gaap_ShortTermDebtTypeDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShortTermDebtTypeAxis" xlink:type="locator" xlink:label="us-gaap_ShortTermDebtTypeAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfShortTermDebtTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfShortTermDebtTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShortTermDebtLineItems" xlink:type="locator" xlink:label="us-gaap_ShortTermDebtLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="us-gaap_ScheduleOfShortTermDebtTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="us-gaap_ShortTermDebtTypeAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ShortTermDebtTypeAxis" xlink:to="us-gaap_ShortTermDebtTypeDomain_2" order="10050.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ShortTermDebtTypeAxis" xlink:to="us-gaap_ShortTermDebtTypeDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtTypeDomain" xlink:to="us-gaap_ConvertibleNotesPayableMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="us-gaap_DebtInstrumentAxis" order="10120.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain_2" order="10130.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" order="10150.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="arct_SecondConvertiblePromissoryNotesMember" order="10170.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="us-gaap_StatementClassOfStockAxis" order="10260.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" order="10290.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_SeriesAPreferredStockMember" order="10310.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="arct_MinimumPercentageOfCapitalStockOwnershipOfInvestorsToWhichConvertibleNotesIssued" order="10520.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="us-gaap_ProceedsFromConvertibleDebt" order="10540.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="us-gaap_DebtInstrumentMaturityDate" order="10560.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateStatedPercentage" order="10580.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="us-gaap_DebtConversionOriginalDebtAmount1" order="10630.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="arct_AccruedInterestOnPromissoryNote" order="10650.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="us-gaap_DebtConversionConvertedInstrumentSharesIssued1" order="10670.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="us-gaap_InducedConversionOfConvertibleDebtExpense" order="10690.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="us-gaap_RepaymentsOfConvertibleDebt" order="10710.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="us-gaap_ConvertibleNotesPayable" order="10730.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="us-gaap_DebtConversionConvertedInstrumentAmount1" order="10880.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="us-gaap_DebtInstrumentConvertibleConversionPrice1" order="10900.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShortTermDebtLineItems" xlink:to="us-gaap_InterestExpense" order="10970.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureShareholdersEquityDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureShareholdersEquityDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureShareholdersEquityDetailsTextual">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:type="locator" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" />
    <loc xlink:href="arct-20171231.xsd#arct_UnvestedRestrictedStockShares" xlink:type="locator" xlink:label="arct_UnvestedRestrictedStockShares" />
    <loc xlink:href="arct-20171231.xsd#arct_NumberOfMilestonesAchieved" xlink:type="locator" xlink:label="arct_NumberOfMilestonesAchieved" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:type="locator" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" />
    <loc xlink:href="arct-20171231.xsd#arct_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionRepurchasePeriodFromTerminationDate" xlink:type="locator" xlink:label="arct_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionRepurchasePeriodFromTerminationDate" />
    <loc xlink:href="arct-20171231.xsd#arct_StockIssuedDuringPeriodSharesSubjectToRepurchaseOption" xlink:type="locator" xlink:label="arct_StockIssuedDuringPeriodSharesSubjectToRepurchaseOption" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharePrice" xlink:type="locator" xlink:label="us-gaap_SharePrice" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:type="locator" xlink:label="us-gaap_CommonStockSharesAuthorized" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" xlink:type="locator" xlink:label="us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquityReverseStockSplit" xlink:type="locator" xlink:label="us-gaap_StockholdersEquityReverseStockSplit" />
    <loc xlink:href="arct-20171231.xsd#arct_ConversionOfStockConversionRatio" xlink:type="locator" xlink:label="arct_ConversionOfStockConversionRatio" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProceedsFromIssuanceOfWarrants" xlink:type="locator" xlink:label="us-gaap_ProceedsFromIssuanceOfWarrants" />
    <loc xlink:href="arct-20171231.xsd#arct_SharesIssuedUponWarrantConversion" xlink:type="locator" xlink:label="arct_SharesIssuedUponWarrantConversion" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:type="locator" xlink:label="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfWarrantOrRightOutstanding" xlink:type="locator" xlink:label="us-gaap_ClassOfWarrantOrRightOutstanding" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" xlink:type="locator" xlink:label="us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:type="locator" xlink:label="us-gaap_PreferredStockSharesOutstanding" />
    <loc xlink:href="arct-20171231.xsd#arct_DebtConversionConvertedInstrumentPricePerShare" xlink:type="locator" xlink:label="arct_DebtConversionConvertedInstrumentPricePerShare" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtConversionConvertedInstrumentSharesIssued1" xlink:type="locator" xlink:label="us-gaap_DebtConversionConvertedInstrumentSharesIssued1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtConversionOriginalDebtAmount1" xlink:type="locator" xlink:label="us-gaap_DebtConversionOriginalDebtAmount1" />
    <loc xlink:href="arct-20171231.xsd#arct_MaximumMaturityTermOfWarrants" xlink:type="locator" xlink:label="arct_MaximumMaturityTermOfWarrants" />
    <loc xlink:href="arct-20171231.xsd#arct_PurchaseOfCommonStockAtStrikePrice" xlink:type="locator" xlink:label="arct_PurchaseOfCommonStockAtStrikePrice" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" xlink:type="locator" xlink:label="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharesIssuedPricePerShare" xlink:type="locator" xlink:label="us-gaap_SharesIssuedPricePerShare" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:type="locator" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" />
    <loc xlink:href="arct-20171231.xsd#arct_RestrictedCommonStockMember" xlink:type="locator" xlink:label="arct_RestrictedCommonStockMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AwardTypeAxis" xlink:type="locator" xlink:label="us-gaap_AwardTypeAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_ShareBasedCompensationAwardTrancheFourMember" xlink:type="locator" xlink:label="arct_ShareBasedCompensationAwardTrancheFourMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationAwardTrancheThreeMember" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationAwardTrancheThreeMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationAwardTrancheTwoMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationAwardTrancheOneMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_VestingDomain" xlink:type="locator" xlink:label="us-gaap_VestingDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_VestingAxis" xlink:type="locator" xlink:label="us-gaap_VestingAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_ArcturusTherapeuticsIncMember" xlink:type="locator" xlink:label="arct_ArcturusTherapeuticsIncMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockMember" xlink:type="locator" xlink:label="us-gaap_CommonStockMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_WarrantMember" xlink:type="locator" xlink:label="us-gaap_WarrantMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EquityComponentDomain" xlink:type="locator" xlink:label="us-gaap_EquityComponentDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:type="locator" xlink:label="us-gaap_StatementEquityComponentsAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_SeriesSeedAndPreferredStockAMember" xlink:type="locator" xlink:label="arct_SeriesSeedAndPreferredStockAMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SeriesAPreferredStockMember" xlink:type="locator" xlink:label="us-gaap_SeriesAPreferredStockMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MaximumMember" xlink:type="locator" xlink:label="us-gaap_MaximumMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MinimumMember" xlink:type="locator" xlink:label="us-gaap_MinimumMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeMember" xlink:type="locator" xlink:label="us-gaap_RangeMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeAxis" xlink:type="locator" xlink:label="us-gaap_RangeAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PrivatePlacementMember" xlink:type="locator" xlink:label="us-gaap_PrivatePlacementMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="locator" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="locator" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:type="locator" xlink:label="us-gaap_SubsidiarySaleOfStockAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_SeriesSeedPreferredStockMember" xlink:type="locator" xlink:label="arct_SeriesSeedPreferredStockMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:type="locator" xlink:label="us-gaap_ClassOfStockDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:type="locator" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfStockByClassTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ClassOfStockLineItems" xlink:type="locator" xlink:label="us-gaap_ClassOfStockLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ScheduleOfStockByClassTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfStockByClassTable" xlink:to="us-gaap_StatementClassOfStockAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="arct_SeriesSeedPreferredStockMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfStockByClassTable" xlink:to="us-gaap_SubsidiarySaleOfStockAxis" order="10100.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain_2" order="10110.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain" order="10130.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_PrivatePlacementMember" order="10150.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfStockByClassTable" xlink:to="us-gaap_RangeAxis" order="10160.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" order="10190.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MinimumMember" order="10210.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MaximumMember" order="10230.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_SeriesAPreferredStockMember" order="10290.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="arct_SeriesSeedAndPreferredStockAMember" order="10470.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfStockByClassTable" xlink:to="us-gaap_StatementEquityComponentsAxis" order="10500.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" order="10530.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_WarrantMember" order="10550.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_CommonStockMember" order="10670.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="arct_ArcturusTherapeuticsIncMember" order="10730.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfStockByClassTable" xlink:to="us-gaap_VestingAxis" order="10740.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_VestingAxis" xlink:to="us-gaap_VestingDomain" order="10770.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="us-gaap_ShareBasedCompensationAwardTrancheOneMember" order="10790.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="us-gaap_ShareBasedCompensationAwardTrancheTwoMember" order="10810.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="us-gaap_ShareBasedCompensationAwardTrancheThreeMember" order="10830.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="arct_ShareBasedCompensationAwardTrancheFourMember" order="10850.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfStockByClassTable" xlink:to="us-gaap_AwardTypeAxis" order="10860.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2" order="10870.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" order="10890.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="arct_RestrictedCommonStockMember" order="10910.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" order="10980.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_SharesIssuedPricePerShare" order="11000.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" order="11020.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="arct_PurchaseOfCommonStockAtStrikePrice" order="11040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="arct_MaximumMaturityTermOfWarrants" order="11060.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_DebtConversionOriginalDebtAmount1" order="11230.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_DebtConversionConvertedInstrumentSharesIssued1" order="11290.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="arct_DebtConversionConvertedInstrumentPricePerShare" order="11350.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_PreferredStockSharesOutstanding" order="11500.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" order="11520.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightOutstanding" order="11570.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" order="11590.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="arct_SharesIssuedUponWarrantConversion" order="11610.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ProceedsFromIssuanceOfWarrants" order="11630.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="arct_ConversionOfStockConversionRatio" order="11680.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_StockholdersEquityReverseStockSplit" order="11730.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" order="11750.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_CommonStockSharesAuthorized" order="11770.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_SharePrice" order="11880.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="arct_StockIssuedDuringPeriodSharesSubjectToRepurchaseOption" order="11940.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="arct_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionRepurchasePeriodFromTerminationDate" order="12000.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" order="12060.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="arct_NumberOfMilestonesAchieved" order="12300.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="arct_UnvestedRestrictedStockShares" order="12360.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" order="12420.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureShareBasedCompensationDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureShareBasedCompensationDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureShareBasedCompensationDetailsTextual">
    <loc xlink:href="arct-20171231.xsd#arct_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableSubjectToRepurchaseConditionsNumber" xlink:type="locator" xlink:label="arct_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableSubjectToRepurchaseConditionsNumber" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:type="locator" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" />
    <loc xlink:href="arct-20171231.xsd#arct_NumberOfBoardOfDirectors" xlink:type="locator" xlink:label="arct_NumberOfBoardOfDirectors" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1" xlink:type="locator" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:type="locator" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:type="locator" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" xlink:type="locator" xlink:label="us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquityReverseStockSplit" xlink:type="locator" xlink:label="us-gaap_StockholdersEquityReverseStockSplit" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance" xlink:type="locator" xlink:label="us-gaap_CommonStockCapitalSharesReservedForFutureIssuance" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" xlink:type="locator" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" />
    <loc xlink:href="arct-20171231.xsd#arct_RestrictedCommonStockMember" xlink:type="locator" xlink:label="arct_RestrictedCommonStockMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DirectorMember" xlink:type="locator" xlink:label="us-gaap_DirectorMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="locator" xlink:label="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="locator" xlink:label="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:type="locator" xlink:label="us-gaap_TitleOfIndividualAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_PerformanceBasedOptionsMember" xlink:type="locator" xlink:label="arct_PerformanceBasedOptionsMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:type="locator" xlink:label="us-gaap_GeneralAndAdministrativeExpenseMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:type="locator" xlink:label="us-gaap_IncomeStatementLocationDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:type="locator" xlink:label="us-gaap_IncomeStatementLocationDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:type="locator" xlink:label="us-gaap_IncomeStatementLocationAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_TwoThousandTenEquityIncentivePlanMember" xlink:type="locator" xlink:label="arct_TwoThousandTenEquityIncentivePlanMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:type="locator" xlink:label="us-gaap_EmployeeStockOptionMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AwardTypeAxis" xlink:type="locator" xlink:label="us-gaap_AwardTypeAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_TwoThousandThirteenEquityIncentivePlanMember" xlink:type="locator" xlink:label="arct_TwoThousandThirteenEquityIncentivePlanMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PlanNameDomain" xlink:type="locator" xlink:label="us-gaap_PlanNameDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PlanNameDomain" xlink:type="locator" xlink:label="us-gaap_PlanNameDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PlanNameAxis" xlink:type="locator" xlink:label="us-gaap_PlanNameAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_PlanNameAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain_2" order="10050.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PlanNameDomain" xlink:to="arct_TwoThousandThirteenEquityIncentivePlanMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" order="10140.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" order="10170.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" order="10190.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PlanNameDomain" xlink:to="arct_TwoThousandTenEquityIncentivePlanMember" order="10290.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_IncomeStatementLocationAxis" order="10400.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain_2" order="10410.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain" order="10430.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="us-gaap_GeneralAndAdministrativeExpenseMember" order="10450.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="arct_PerformanceBasedOptionsMember" order="10510.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_TitleOfIndividualAxis" order="10600.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_TitleOfIndividualAxis" xlink:to="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_2" order="10610.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TitleOfIndividualAxis" xlink:to="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" order="10630.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="us-gaap_DirectorMember" order="10650.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="arct_RestrictedCommonStockMember" order="10710.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" order="10740.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" order="10760.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_CommonStockCapitalSharesReservedForFutureIssuance" order="10780.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_StockholdersEquityReverseStockSplit" order="10830.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" order="10870.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" order="11020.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" order="11080.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1" order="11140.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" order="11230.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="arct_NumberOfBoardOfDirectors" order="11250.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" order="11270.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="arct_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableSubjectToRepurchaseConditionsNumber" order="11360.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureShareBasedCompensationDetails" xlink:href="arct-20171231.xsd#Role_DisclosureShareBasedCompensationDetails" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureShareBasedCompensationDetails">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:type="locator" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:type="locator" xlink:label="us-gaap_EmployeeStockOptionMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AwardTypeAxis" xlink:type="locator" xlink:label="us-gaap_AwardTypeAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" order="10180.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" order="10240.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" order="10300.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" order="10360.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" order="10420.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureShareBasedCompensationDetails2" xlink:href="arct-20171231.xsd#Role_DisclosureShareBasedCompensationDetails2" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureShareBasedCompensationDetails2">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:type="locator" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:type="locator" xlink:label="us-gaap_GeneralAndAdministrativeExpenseMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:type="locator" xlink:label="us-gaap_ResearchAndDevelopmentExpenseMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:type="locator" xlink:label="us-gaap_IncomeStatementLocationDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:type="locator" xlink:label="us-gaap_IncomeStatementLocationAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_IncomeStatementLocationAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="us-gaap_ResearchAndDevelopmentExpenseMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="us-gaap_GeneralAndAdministrativeExpenseMember" order="10110.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" order="10140.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureIncomeTaxesDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureIncomeTaxesDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureIncomeTaxesDetailsTextual">
    <loc xlink:href="arct-20171231.xsd#arct_TaxCutsAndJobsActOf2017ChangeInTaxRateReductionInDeferredTaxAssets" xlink:type="locator" xlink:label="arct_TaxCutsAndJobsActOf2017ChangeInTaxRateReductionInDeferredTaxAssets" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:type="locator" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" />
    <loc xlink:href="arct-20171231.xsd#arct_NetOperatingLossAndResearchAndDevelopmentCreditCarryforwardsLimitationsOnCumulativeChangeInOwnershipPeriod" xlink:type="locator" xlink:label="arct_NetOperatingLossAndResearchAndDevelopmentCreditCarryforwardsLimitationsOnCumulativeChangeInOwnershipPeriod" />
    <loc xlink:href="arct-20171231.xsd#arct_NetOperatingLossAndResearchAndDevelopmentCreditCarryforwardsLimitationsOnCumulativeChangeInOwnershipMinimumPercentage" xlink:type="locator" xlink:label="arct_NetOperatingLossAndResearchAndDevelopmentCreditCarryforwardsLimitationsOnCumulativeChangeInOwnershipMinimumPercentage" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:type="locator" xlink:label="us-gaap_ResearchAndDevelopmentExpense" />
    <loc xlink:href="arct-20171231.xsd#arct_TaxCreditCarryforwardExpirationYear" xlink:type="locator" xlink:label="arct_TaxCreditCarryforwardExpirationYear" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TaxCreditCarryforwardAmount" xlink:type="locator" xlink:label="us-gaap_TaxCreditCarryforwardAmount" />
    <loc xlink:href="arct-20171231.xsd#arct_OperatingLossCarryforwardsExpirationYear" xlink:type="locator" xlink:label="arct_OperatingLossCarryforwardsExpirationYear" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OperatingLossCarryforwards" xlink:type="locator" xlink:label="us-gaap_OperatingLossCarryforwards" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount" xlink:type="locator" xlink:label="us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense" xlink:type="locator" xlink:label="us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate" xlink:type="locator" xlink:label="us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_UnrecognizedTaxBenefits" xlink:type="locator" xlink:label="us-gaap_UnrecognizedTaxBenefits" />
    <loc xlink:href="arct-20171231.xsd#arct_MaximumPercentageOfLikelihoodOfUncertainTaxPositionRecognizedToBeSustained" xlink:type="locator" xlink:label="arct_MaximumPercentageOfLikelihoodOfUncertainTaxPositionRecognizedToBeSustained" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MinimumMember" xlink:type="locator" xlink:label="us-gaap_MinimumMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MaximumMember" xlink:type="locator" xlink:label="us-gaap_MaximumMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeMember" xlink:type="locator" xlink:label="us-gaap_RangeMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeAxis" xlink:type="locator" xlink:label="us-gaap_RangeAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScenarioPlanMember" xlink:type="locator" xlink:label="us-gaap_ScenarioPlanMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator" xlink:label="us-gaap_ScenarioUnspecifiedDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator" xlink:label="us-gaap_ScenarioUnspecifiedDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:type="locator" xlink:label="us-gaap_StatementScenarioAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ResearchMember" xlink:type="locator" xlink:label="us-gaap_ResearchMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TaxCreditCarryforwardNameDomain" xlink:type="locator" xlink:label="us-gaap_TaxCreditCarryforwardNameDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TaxCreditCarryforwardNameDomain" xlink:type="locator" xlink:label="us-gaap_TaxCreditCarryforwardNameDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TaxCreditCarryforwardAxis" xlink:type="locator" xlink:label="us-gaap_TaxCreditCarryforwardAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ForeignCountryMember" xlink:type="locator" xlink:label="us-gaap_ForeignCountryMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StateAndLocalJurisdictionMember" xlink:type="locator" xlink:label="us-gaap_StateAndLocalJurisdictionMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DomesticCountryMember" xlink:type="locator" xlink:label="us-gaap_DomesticCountryMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:type="locator" xlink:label="us-gaap_IncomeTaxAuthorityDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:type="locator" xlink:label="us-gaap_IncomeTaxAuthorityDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:type="locator" xlink:label="us-gaap_IncomeTaxAuthorityAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_AlcobraLimitedMember" xlink:type="locator" xlink:label="arct_AlcobraLimitedMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <loc xlink:href="http://xbrl.sec.gov/stpr/2011/stpr-2011-01-31.xsd#stpr_CA" xlink:type="locator" xlink:label="stpr_CA" />
    <loc xlink:href="http://xbrl.sec.gov/country/2017/country-2017-01-31.xsd#country_US" xlink:type="locator" xlink:label="country_US" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:type="locator" xlink:label="us-gaap_SegmentGeographicalDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:type="locator" xlink:label="us-gaap_SegmentGeographicalDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:type="locator" xlink:label="us-gaap_StatementGeographicalAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_IncomeTaxesTable" xlink:type="locator" xlink:label="arct_IncomeTaxesTable" />
    <loc xlink:href="arct-20171231.xsd#arct_IncomeTaxesLineItems" xlink:type="locator" xlink:label="arct_IncomeTaxesLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="arct_IncomeTaxesLineItems" xlink:to="arct_IncomeTaxesTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_IncomeTaxesTable" xlink:to="us-gaap_StatementGeographicalAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementGeographicalAxis" xlink:to="us-gaap_SegmentGeographicalDomain_2" order="10050.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementGeographicalAxis" xlink:to="us-gaap_SegmentGeographicalDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentGeographicalDomain" xlink:to="country_US" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentGeographicalDomain" xlink:to="stpr_CA" order="10110.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_IncomeTaxesTable" xlink:to="us-gaap_BusinessAcquisitionAxis" order="10120.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" order="10150.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="arct_AlcobraLimitedMember" order="10170.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_IncomeTaxesTable" xlink:to="us-gaap_IncomeTaxAuthorityAxis" order="10180.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeTaxAuthorityAxis" xlink:to="us-gaap_IncomeTaxAuthorityDomain_2" order="10190.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeTaxAuthorityAxis" xlink:to="us-gaap_IncomeTaxAuthorityDomain" order="10210.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeTaxAuthorityDomain" xlink:to="us-gaap_DomesticCountryMember" order="10230.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeTaxAuthorityDomain" xlink:to="us-gaap_StateAndLocalJurisdictionMember" order="10250.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeTaxAuthorityDomain" xlink:to="us-gaap_ForeignCountryMember" order="10270.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_IncomeTaxesTable" xlink:to="us-gaap_TaxCreditCarryforwardAxis" order="10480.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_TaxCreditCarryforwardAxis" xlink:to="us-gaap_TaxCreditCarryforwardNameDomain_2" order="10490.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TaxCreditCarryforwardAxis" xlink:to="us-gaap_TaxCreditCarryforwardNameDomain" order="10510.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TaxCreditCarryforwardNameDomain" xlink:to="us-gaap_ResearchMember" order="10530.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_IncomeTaxesTable" xlink:to="us-gaap_StatementScenarioAxis" order="10600.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain_2" order="10610.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" order="10630.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScenarioUnspecifiedDomain" xlink:to="us-gaap_ScenarioPlanMember" order="10650.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_IncomeTaxesTable" xlink:to="us-gaap_RangeAxis" order="10660.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" order="10690.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MaximumMember" order="10710.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MinimumMember" order="10730.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="arct_MaximumPercentageOfLikelihoodOfUncertainTaxPositionRecognizedToBeSustained" order="10760.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="us-gaap_UnrecognizedTaxBenefits" order="10810.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate" order="10870.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense" order="11060.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount" order="11190.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="us-gaap_OperatingLossCarryforwards" order="11240.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="arct_OperatingLossCarryforwardsExpirationYear" order="11380.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="us-gaap_TaxCreditCarryforwardAmount" order="11550.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="arct_TaxCreditCarryforwardExpirationYear" order="11630.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="us-gaap_ResearchAndDevelopmentExpense" order="11740.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="arct_NetOperatingLossAndResearchAndDevelopmentCreditCarryforwardsLimitationsOnCumulativeChangeInOwnershipMinimumPercentage" order="11810.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="arct_NetOperatingLossAndResearchAndDevelopmentCreditCarryforwardsLimitationsOnCumulativeChangeInOwnershipPeriod" order="11830.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" order="11880.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_IncomeTaxesLineItems" xlink:to="arct_TaxCutsAndJobsActOf2017ChangeInTaxRateReductionInDeferredTaxAssets" order="11920.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureCommitmentsAndContingenciesDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureCommitmentsAndContingenciesDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureCommitmentsAndContingenciesDetailsTextual">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongTermPurchaseCommitmentAmount" xlink:type="locator" xlink:label="us-gaap_LongTermPurchaseCommitmentAmount" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OperatingLeasesRentExpenseNet" xlink:type="locator" xlink:label="us-gaap_OperatingLeasesRentExpenseNet" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OperatingLeasesIncomeStatementSubleaseRevenue" xlink:type="locator" xlink:label="us-gaap_OperatingLeasesIncomeStatementSubleaseRevenue" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LeaseExpirationDate1" xlink:type="locator" xlink:label="us-gaap_LeaseExpirationDate1" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SecurityDeposit" xlink:type="locator" xlink:label="us-gaap_SecurityDeposit" />
    <loc xlink:href="arct-20171231.xsd#arct_LessorLeasingArrangementsOperatingLeasesExtendedLease" xlink:type="locator" xlink:label="arct_LessorLeasingArrangementsOperatingLeasesExtendedLease" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PaymentsForProceedsFromTenantAllowance" xlink:type="locator" xlink:label="us-gaap_PaymentsForProceedsFromTenantAllowance" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LessorLeasingArrangementsOperatingLeasesTermOfContract" xlink:type="locator" xlink:label="us-gaap_LessorLeasingArrangementsOperatingLeasesTermOfContract" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LessorLeasingArrangementsOperatingLeasesRenewalTerm" xlink:type="locator" xlink:label="us-gaap_LessorLeasingArrangementsOperatingLeasesRenewalTerm" />
    <loc xlink:href="arct-20171231.xsd#arct_LessorLeasingArrangementsOperatingLeasesExpirationPeriodYearAndMonth" xlink:type="locator" xlink:label="arct_LessorLeasingArrangementsOperatingLeasesExpirationPeriodYearAndMonth" />
    <loc xlink:href="arct-20171231.xsd#arct_LessorLeasingArrangementsOperatingLeasesInitialPeriodYearAndMonth" xlink:type="locator" xlink:label="arct_LessorLeasingArrangementsOperatingLeasesInitialPeriodYearAndMonth" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NetRentableArea" xlink:type="locator" xlink:label="us-gaap_NetRentableArea" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_GrantsReceivableCurrent" xlink:type="locator" xlink:label="us-gaap_GrantsReceivableCurrent" />
    <loc xlink:href="arct-20171231.xsd#arct_OctoberTwoThousandAndSeventeenLeaseAmendmentMember" xlink:type="locator" xlink:label="arct_OctoberTwoThousandAndSeventeenLeaseAmendmentMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LeaseArrangementTypeDomain" xlink:type="locator" xlink:label="us-gaap_LeaseArrangementTypeDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LeaseArrangementTypeDomain" xlink:type="locator" xlink:label="us-gaap_LeaseArrangementTypeDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LeaseArrangementTypeAxis" xlink:type="locator" xlink:label="us-gaap_LeaseArrangementTypeAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventMember" xlink:type="locator" xlink:label="us-gaap_SubsequentEventMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:type="locator" xlink:label="us-gaap_SubsequentEventTypeDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:type="locator" xlink:label="us-gaap_SubsequentEventTypeDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:type="locator" xlink:label="us-gaap_SubsequentEventTypeAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertySubjectToOperatingLeaseMember" xlink:type="locator" xlink:label="us-gaap_PropertySubjectToOperatingLeaseMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertySubjectToOrAvailableForOperatingLeaseDomain" xlink:type="locator" xlink:label="us-gaap_PropertySubjectToOrAvailableForOperatingLeaseDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertySubjectToOrAvailableForOperatingLeaseDomain" xlink:type="locator" xlink:label="us-gaap_PropertySubjectToOrAvailableForOperatingLeaseDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis" xlink:type="locator" xlink:label="us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_SanDiegoCaliforniaMember" xlink:type="locator" xlink:label="arct_SanDiegoCaliforniaMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:type="locator" xlink:label="us-gaap_SegmentGeographicalDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:type="locator" xlink:label="us-gaap_StatementGeographicalAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_CFFTGrantsMember" xlink:type="locator" xlink:label="arct_CFFTGrantsMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:type="locator" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:type="locator" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:type="locator" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_CommitmentAndContingenciesTable" xlink:type="locator" xlink:label="arct_CommitmentAndContingenciesTable" />
    <loc xlink:href="arct-20171231.xsd#arct_CommitmentAndContingenciesLineItems" xlink:type="locator" xlink:label="arct_CommitmentAndContingenciesLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="arct_CommitmentAndContingenciesTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_CommitmentAndContingenciesTable" xlink:to="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:to="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_2" order="10050.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:to="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:to="arct_CFFTGrantsMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_CommitmentAndContingenciesTable" xlink:to="us-gaap_StatementGeographicalAxis" order="10100.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementGeographicalAxis" xlink:to="us-gaap_SegmentGeographicalDomain" order="10130.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentGeographicalDomain" xlink:to="arct_SanDiegoCaliforniaMember" order="10150.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_CommitmentAndContingenciesTable" xlink:to="us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis" order="10240.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis" xlink:to="us-gaap_PropertySubjectToOrAvailableForOperatingLeaseDomain_2" order="10250.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis" xlink:to="us-gaap_PropertySubjectToOrAvailableForOperatingLeaseDomain" order="10270.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertySubjectToOrAvailableForOperatingLeaseDomain" xlink:to="us-gaap_PropertySubjectToOperatingLeaseMember" order="10290.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_CommitmentAndContingenciesTable" xlink:to="us-gaap_SubsequentEventTypeAxis" order="10300.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_SubsequentEventTypeAxis" xlink:to="us-gaap_SubsequentEventTypeDomain_2" order="10310.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsequentEventTypeAxis" xlink:to="us-gaap_SubsequentEventTypeDomain" order="10330.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsequentEventTypeDomain" xlink:to="us-gaap_SubsequentEventMember" order="10350.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="arct_CommitmentAndContingenciesTable" xlink:to="us-gaap_LeaseArrangementTypeAxis" order="10360.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LeaseArrangementTypeAxis" xlink:to="us-gaap_LeaseArrangementTypeDomain_2" order="10370.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LeaseArrangementTypeAxis" xlink:to="us-gaap_LeaseArrangementTypeDomain" order="10390.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LeaseArrangementTypeDomain" xlink:to="arct_OctoberTwoThousandAndSeventeenLeaseAmendmentMember" order="10410.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="us-gaap_GrantsReceivableCurrent" order="10560.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="us-gaap_NetRentableArea" order="10610.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="arct_LessorLeasingArrangementsOperatingLeasesInitialPeriodYearAndMonth" order="10630.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="arct_LessorLeasingArrangementsOperatingLeasesExpirationPeriodYearAndMonth" order="10650.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="us-gaap_LessorLeasingArrangementsOperatingLeasesRenewalTerm" order="10670.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="us-gaap_LessorLeasingArrangementsOperatingLeasesTermOfContract" order="10750.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="us-gaap_PaymentsForProceedsFromTenantAllowance" order="10770.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="arct_LessorLeasingArrangementsOperatingLeasesExtendedLease" order="10790.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="us-gaap_SecurityDeposit" order="10810.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="us-gaap_LeaseExpirationDate1" order="10830.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="us-gaap_OperatingLeasesIncomeStatementSubleaseRevenue" order="10850.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="us-gaap_OperatingLeasesRentExpenseNet" order="10920.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="arct_CommitmentAndContingenciesLineItems" xlink:to="us-gaap_LongTermPurchaseCommitmentAmount" order="11010.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureRelatedPartyTransactionsAdditionalInformationDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureRelatedPartyTransactionsAdditionalInformationDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureRelatedPartyTransactionsAdditionalInformationDetailsTextual">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners" xlink:type="locator" xlink:label="us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensation" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensation" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccountsReceivableRelatedParties" xlink:type="locator" xlink:label="us-gaap_AccountsReceivableRelatedParties" />
    <loc xlink:href="arct-20171231.xsd#arct_RevenueRelatedToAmortizationOfUpfrontPayment" xlink:type="locator" xlink:label="arct_RevenueRelatedToAmortizationOfUpfrontPayment" />
    <loc xlink:href="arct-20171231.xsd#arct_UpfrontFeeReceived" xlink:type="locator" xlink:label="arct_UpfrontFeeReceived" />
    <loc xlink:href="arct-20171231.xsd#arct_PresidentAndChiefExecutiveOfficerMember" xlink:type="locator" xlink:label="arct_PresidentAndChiefExecutiveOfficerMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="locator" xlink:label="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:type="locator" xlink:label="us-gaap_TitleOfIndividualAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_ResearchCollaborationAndLicenseAgreementMember" xlink:type="locator" xlink:label="arct_ResearchCollaborationAndLicenseAgreementMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:type="locator" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:type="locator" xlink:label="us-gaap_TypeOfArrangementAxis" />
    <loc xlink:href="arct-20171231.xsd#arct_ProvidenceTherapeuticsIncorporationMember" xlink:type="locator" xlink:label="arct_ProvidenceTherapeuticsIncorporationMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:type="locator" xlink:label="us-gaap_RelatedPartyDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:type="locator" xlink:label="us-gaap_RelatedPartyDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:type="locator" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:type="locator" xlink:label="us-gaap_RelatedPartyTransactionLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain_2" order="10050.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="arct_ProvidenceTherapeuticsIncorporationMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="us-gaap_TypeOfArrangementAxis" order="10180.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember" order="10210.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="arct_ResearchCollaborationAndLicenseAgreementMember" order="10230.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="us-gaap_TitleOfIndividualAxis" order="10320.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TitleOfIndividualAxis" xlink:to="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" order="10350.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="arct_PresidentAndChiefExecutiveOfficerMember" order="10370.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="arct_UpfrontFeeReceived" order="10400.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="arct_RevenueRelatedToAmortizationOfUpfrontPayment" order="10440.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_AccountsReceivableRelatedParties" order="10480.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ShareBasedCompensation" order="10520.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners" order="10560.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureLitigationDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureLitigationDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureLitigationDetailsTextual">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LossContingencyAllegations" xlink:type="locator" xlink:label="us-gaap_LossContingencyAllegations" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LossContingencyNameOfDefendant" xlink:type="locator" xlink:label="us-gaap_LossContingencyNameOfDefendant" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LossContingencyNameOfPlaintiff" xlink:type="locator" xlink:label="us-gaap_LossContingencyNameOfPlaintiff" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LossContingencyLawsuitFilingDate" xlink:type="locator" xlink:label="us-gaap_LossContingencyLawsuitFilingDate" />
    <loc xlink:href="arct-20171231.xsd#arct_CaliforniaFederalCourtLitigationMember" xlink:type="locator" xlink:label="arct_CaliforniaFederalCourtLitigationMember" />
    <loc xlink:href="arct-20171231.xsd#arct_CaliforniaStateCourtLitigationMember" xlink:type="locator" xlink:label="arct_CaliforniaStateCourtLitigationMember" />
    <loc xlink:href="arct-20171231.xsd#arct_IsraelLitigationMember" xlink:type="locator" xlink:label="arct_IsraelLitigationMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LitigationCaseTypeDomain" xlink:type="locator" xlink:label="us-gaap_LitigationCaseTypeDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LitigationCaseTypeDomain" xlink:type="locator" xlink:label="us-gaap_LitigationCaseTypeDomain_2" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LitigationCaseAxis" xlink:type="locator" xlink:label="us-gaap_LitigationCaseAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LossContingenciesTable" xlink:type="locator" xlink:label="us-gaap_LossContingenciesTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:type="locator" xlink:label="us-gaap_LossContingenciesLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_LossContingenciesLineItems" xlink:to="us-gaap_LossContingenciesTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_LossContingenciesTable" xlink:to="us-gaap_LitigationCaseAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LitigationCaseAxis" xlink:to="us-gaap_LitigationCaseTypeDomain_2" order="10050.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LitigationCaseAxis" xlink:to="us-gaap_LitigationCaseTypeDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LitigationCaseTypeDomain" xlink:to="arct_IsraelLitigationMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LitigationCaseTypeDomain" xlink:to="arct_CaliforniaStateCourtLitigationMember" order="10150.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LitigationCaseTypeDomain" xlink:to="arct_CaliforniaFederalCourtLitigationMember" order="10210.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LossContingenciesLineItems" xlink:to="us-gaap_LossContingencyLawsuitFilingDate" order="10240.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LossContingenciesLineItems" xlink:to="us-gaap_LossContingencyNameOfPlaintiff" order="10260.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LossContingenciesLineItems" xlink:to="us-gaap_LossContingencyNameOfDefendant" order="10280.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LossContingenciesLineItems" xlink:to="us-gaap_LossContingencyAllegations" order="10300.00" priority="2" use="optional" />
  </definitionLink>
  <roleRef roleURI="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureSubsequentEventsDetailsTextual" xlink:href="arct-20171231.xsd#Role_DisclosureSubsequentEventsDetailsTextual" xlink:type="simple" />
  <definitionLink xlink:type="extended" xlink:role="http://www.alcobra-pharma.com/20171231/taxonomy/role/DisclosureSubsequentEventsDetailsTextual">
    <loc xlink:href="arct-20171231.xsd#arct_CashPaymentForScientificAdvisoryServices" xlink:type="locator" xlink:label="arct_CashPaymentForScientificAdvisoryServices" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesIssuedForServices" xlink:type="locator" xlink:label="us-gaap_StockIssuedDuringPeriodSharesIssuedForServices" />
    <loc xlink:href="arct-20171231.xsd#arct_CashPayment" xlink:type="locator" xlink:label="arct_CashPayment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventMember" xlink:type="locator" xlink:label="us-gaap_SubsequentEventMember" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:type="locator" xlink:label="us-gaap_SubsequentEventTypeDomain" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:type="locator" xlink:label="us-gaap_SubsequentEventTypeAxis" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTable" xlink:type="locator" xlink:label="us-gaap_SubsequentEventTable" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventLineItems" xlink:type="locator" xlink:label="us-gaap_SubsequentEventLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="us-gaap_SubsequentEventTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_SubsequentEventTypeAxis" order="10040.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsequentEventTypeAxis" xlink:to="us-gaap_SubsequentEventTypeDomain" order="10070.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsequentEventTypeDomain" xlink:to="us-gaap_SubsequentEventMember" order="10090.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="arct_CashPayment" order="10120.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesIssuedForServices" order="10140.00" priority="2" use="optional" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="arct_CashPaymentForScientificAdvisoryServices" order="10160.00" priority="2" use="optional" />
  </definitionLink>
</linkbase>





