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    <link:label id="lab_us-gaap_BalanceSheetLocationDomain_A1F6B6AA56B922B30A86F28E175AB655_terseLabel_en-US" xlink:label="lab_us-gaap_BalanceSheetLocationDomain_A1F6B6AA56B922B30A86F28E175AB655" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Balance Sheet Location [Domain]</link:label>
    <link:label id="lab_us-gaap_BalanceSheetLocationDomain_A1F6B6AA56B922B30A86F28E175AB655_label_en-US" xlink:label="lab_us-gaap_BalanceSheetLocationDomain_A1F6B6AA56B922B30A86F28E175AB655" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Balance Sheet Location [Domain]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_A1F6B6AA56B922B30A86F28E175AB655" xlink:type="locator" />
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    <link:label id="lab_us-gaap_SellingGeneralAndAdministrativeExpensesMember_5DB47DBA2D2766F3F224F28E175ACB09_label_en-US" xlink:label="lab_us-gaap_SellingGeneralAndAdministrativeExpensesMember_5DB47DBA2D2766F3F224F28E175ACB09" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Selling, General and Administrative Expenses [Member]</link:label>
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    <link:label id="lab_us-gaap_EmployeeSeveranceMember_14ACF0D9963953A74E66F28E175A55BA_terseLabel_en-US" xlink:label="lab_us-gaap_EmployeeSeveranceMember_14ACF0D9963953A74E66F28E175A55BA" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Employee termination benefits</link:label>
    <link:label id="lab_us-gaap_EmployeeSeveranceMember_14ACF0D9963953A74E66F28E175A55BA_label_en-US" xlink:label="lab_us-gaap_EmployeeSeveranceMember_14ACF0D9963953A74E66F28E175A55BA" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Employee Severance [Member]</link:label>
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    <link:label id="lab_us-gaap_OneTimeTerminationBenefitsMember_B634C7B38281DFF77070F28E175A26B1_label_en-US" xlink:label="lab_us-gaap_OneTimeTerminationBenefitsMember_B634C7B38281DFF77070F28E175A26B1" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">One-time Termination Benefits [Member]</link:label>
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    <link:label id="lab_flks_RetentionBenefitsMember_768DE3944AF5CDF32EF7F28E175AB96A_terseLabel_en-US" xlink:label="lab_flks_RetentionBenefitsMember_768DE3944AF5CDF32EF7F28E175AB96A" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Employee retention benefits</link:label>
    <link:label id="lab_flks_RetentionBenefitsMember_768DE3944AF5CDF32EF7F28E175AB96A_label_en-US" xlink:label="lab_flks_RetentionBenefitsMember_768DE3944AF5CDF32EF7F28E175AB96A" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Retention Benefits [Member]</link:label>
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    <link:loc xlink:href="flks-20180331.xsd#flks_RetentionBenefitsMember" xlink:label="loc_flks_RetentionBenefitsMember_768DE3944AF5CDF32EF7F28E175AB96A" xlink:type="locator" />
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    <link:label id="lab_flks_AccruedResearchAndDevelopmentExpenseCurrent_B5A96BCA49023A367AA1F28E175ECC5D_terseLabel_en-US" xlink:label="lab_flks_AccruedResearchAndDevelopmentExpenseCurrent_B5A96BCA49023A367AA1F28E175ECC5D" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Accrued research and development expense</link:label>
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    <link:label id="lab_flks_PercentageDecreaseinCompanyWorkforce_6E39C893CD85DDCE73A9F28E175E8791_negatedTerseLabel_en-US" xlink:label="lab_flks_PercentageDecreaseinCompanyWorkforce_6E39C893CD85DDCE73A9F28E175E8791" xlink:role="http://www.xbrl.org/2009/role/negatedTerseLabel" xlink:type="resource" xml:lang="en-US">Percentage decrease in company workforce</link:label>
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