<?xml version="1.0" encoding="us-ascii"?>
<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2021-05-07T17:06:43.2624717+00:00 -->
<!-- Version            : 5.0.1.615 -->
<!-- Package ID         : 408bd17595454c83bf1869d6bdabac52 -->
<!-- Copyright (c) 2021 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="algs_StockIssuesEarlyPeriodShareStockOptionsExercised" name="StockIssuesEarlyPeriodShareStockOptionsExercised" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="algs_AggregateShareRepurchasePrice" name="AggregateShareRepurchasePrice" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="algs_FinancialInstrumentsFinancialAssetsAndLiabilitiesBalanceSheetGroupingsAbstract" name="FinancialInstrumentsFinancialAssetsAndLiabilitiesBalanceSheetGroupingsAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="algs_UpfrontLicenseFeesPaid" name="UpfrontLicenseFeesPaid" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="algs_EmoryLicenseAgreementMember" name="EmoryLicenseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="algs_EmoryUniversityMember" name="EmoryUniversityMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="algs_LuxnaLicenseAgreementMember" name="LuxnaLicenseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="algs_LuxnaBiotechCoLtdMember" name="LuxnaBiotechCoLtdMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="algs_KatholiekeUniversiteitLeuvenLicenseAgreementMember" name="KatholiekeUniversiteitLeuvenLicenseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="algs_KULeuvenMember" name="KULeuvenMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="algs_KatholiekeUniversiteitLeuvenMember" name="KatholiekeUniversiteitLeuvenMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="algs_MerckLicenseAndResearchCollaborationMember" name="MerckLicenseAndResearchCollaborationMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="algs_MerckMember" name="MerckMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="algs_LongTermPurchaseCommitmentAdditionalObligationToBePaid" name="LongTermPurchaseCommitmentAdditionalObligationToBePaid" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="algs_ResearchAndDevelopmentArrangementContractToPerformForOthersExpiryPeriod" name="ResearchAndDevelopmentArrangementContractToPerformForOthersExpiryPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="algs_LongtermPurchaseCommitmentPeriodDescription" name="LongtermPurchaseCommitmentPeriodDescription" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="algs_ExpenseRelatedToMilestonePayments" name="ExpenseRelatedToMilestonePayments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="algs_ExpenseOrAccrualsRecognizedRelatedToRoyalties" name="ExpenseOrAccrualsRecognizedRelatedToRoyalties" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="algs_FirstCommercialSaleOfALicensedProductPeriod" name="FirstCommercialSaleOfALicensedProductPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="algs_MilestonePaymentsAndRoyaltiesReceivable" name="MilestonePaymentsAndRoyaltiesReceivable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
</xsd:schema>
