<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019ff692-1efa-75bd-8822-3421d40f1adc,g:1c7de97a-cee2-4e4e-b912-db33a578e774-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 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">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.mineralystx.com/role/CondensedStatementsofStockholdersEquity" xlink:type="simple" xlink:href="mlys-20260930.xsd#CondensedStatementsofStockholdersEquity"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/CondensedStatementsofStockholdersEquity" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019ff692-1fa9-7c40-99e4-2dd0e1069c66" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019ff692-1fa9-76c5-a6a9-ad0a5f43dfaf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019ff692-1fa9-7c40-99e4-2dd0e1069c66" xlink:to="loc_us-gaap_StatementTable_019ff692-1fa9-76c5-a6a9-ad0a5f43dfaf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019ff692-1fa9-75c4-b742-8cfe1b513847" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ff692-1fa9-76c5-a6a9-ad0a5f43dfaf" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019ff692-1fa9-75c4-b742-8cfe1b513847" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ff692-1fa9-75c4-b742-8cfe1b513847_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ff692-1fa9-75c4-b742-8cfe1b513847" xlink:to="loc_us-gaap_EquityComponentDomain_019ff692-1fa9-75c4-b742-8cfe1b513847_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ff692-1fa9-70dc-ba27-aa2302c78dc5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ff692-1fa9-75c4-b742-8cfe1b513847" xlink:to="loc_us-gaap_EquityComponentDomain_019ff692-1fa9-70dc-ba27-aa2302c78dc5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019ff692-1fa9-7b86-bf2f-74e1a73039b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ff692-1fa9-70dc-ba27-aa2302c78dc5" xlink:to="loc_us-gaap_CommonStockMember_019ff692-1fa9-7b86-bf2f-74e1a73039b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019ff692-1fa9-716c-bd04-8cf9c184d286" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ff692-1fa9-70dc-ba27-aa2302c78dc5" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019ff692-1fa9-716c-bd04-8cf9c184d286" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019ff692-1fa9-74e9-a2a9-fedb3459ca88" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ff692-1fa9-70dc-ba27-aa2302c78dc5" xlink:to="loc_us-gaap_RetainedEarningsMember_019ff692-1fa9-74e9-a2a9-fedb3459ca88" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1fa9-7ac8-a413-ff87820db5a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ff692-1fa9-76c5-a6a9-ad0a5f43dfaf" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1fa9-7ac8-a413-ff87820db5a5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-7ac8-a413-ff87820db5a5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1fa9-7ac8-a413-ff87820db5a5" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-7ac8-a413-ff87820db5a5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-71f6-b2ed-8d556de12ccb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1fa9-7ac8-a413-ff87820db5a5" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-71f6-b2ed-8d556de12ccb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_PublicStockOfferingMember_019ff692-1fa9-7d48-8f37-294f8737b12d" xlink:href="mlys-20260930.xsd#mlys_PublicStockOfferingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-71f6-b2ed-8d556de12ccb" xlink:to="loc_mlys_PublicStockOfferingMember_019ff692-1fa9-7d48-8f37-294f8737b12d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_AtTheMarketAgreementMember_019ff692-1fa9-7473-aab1-f454506e388e" xlink:href="mlys-20260930.xsd#mlys_AtTheMarketAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-71f6-b2ed-8d556de12ccb" xlink:to="loc_mlys_AtTheMarketAgreementMember_019ff692-1fa9-7473-aab1-f454506e388e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ff692-1fa9-75ae-a0ef-a5c7eec21051" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ff692-1fa9-7c40-99e4-2dd0e1069c66" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ff692-1fa9-75ae-a0ef-a5c7eec21051" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019ff692-1fa9-7d07-8053-24c558e4ddab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ff692-1fa9-75ae-a0ef-a5c7eec21051" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019ff692-1fa9-7d07-8053-24c558e4ddab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019ff692-1fa9-7dec-98a4-d65ff679fe9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ff692-1fa9-75ae-a0ef-a5c7eec21051" xlink:to="loc_us-gaap_StockholdersEquity_019ff692-1fa9-7dec-98a4-d65ff679fe9c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019ff692-1fa9-7dbc-bfe3-bbf7b476c5f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ff692-1fa9-75ae-a0ef-a5c7eec21051" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019ff692-1fa9-7dbc-bfe3-bbf7b476c5f1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019ff692-1fa9-7eb7-93d3-48c54ab85a01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ff692-1fa9-75ae-a0ef-a5c7eec21051" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019ff692-1fa9-7eb7-93d3-48c54ab85a01" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019ff692-1fa9-7de5-a304-9e9f1fab4467" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ff692-1fa9-75ae-a0ef-a5c7eec21051" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019ff692-1fa9-7de5-a304-9e9f1fab4467" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019ff692-1fa9-7e09-9559-48b70d44576c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ff692-1fa9-75ae-a0ef-a5c7eec21051" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019ff692-1fa9-7e09-9559-48b70d44576c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019ff692-1fa9-7b25-8937-7ecb8b1de2dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ff692-1fa9-75ae-a0ef-a5c7eec21051" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019ff692-1fa9-7b25-8937-7ecb8b1de2dc" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019ff692-1fa9-7cf4-8d57-233f2f6b5360" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ff692-1fa9-75ae-a0ef-a5c7eec21051" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019ff692-1fa9-7cf4-8d57-233f2f6b5360" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019ff692-1fa9-7246-bdbc-7e88987b2e86" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ff692-1fa9-75ae-a0ef-a5c7eec21051" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019ff692-1fa9-7246-bdbc-7e88987b2e86" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019ff692-1fa9-7476-b368-42b68487a34c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019ff692-1fa9-75ae-a0ef-a5c7eec21051" xlink:to="loc_us-gaap_NetIncomeLoss_019ff692-1fa9-7476-b368-42b68487a34c" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019ff692-1fa9-7519-af2e-a9d604027aa2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019ff692-1fa9-7c34-ba92-cf3064ca7e56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/CondensedStatementsofStockholdersEquityParenthetical" xlink:type="simple" xlink:href="mlys-20260930.xsd#CondensedStatementsofStockholdersEquityParenthetical"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/CondensedStatementsofStockholdersEquityParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019ff692-1fa9-71ea-8d3f-f3682f1e969c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019ff692-1fa9-78bc-95bc-c9bb395c0d17" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019ff692-1fa9-71ea-8d3f-f3682f1e969c" xlink:to="loc_us-gaap_StatementTable_019ff692-1fa9-78bc-95bc-c9bb395c0d17" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1fa9-7249-a1ad-01b39d917651" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ff692-1fa9-78bc-95bc-c9bb395c0d17" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1fa9-7249-a1ad-01b39d917651" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-7249-a1ad-01b39d917651_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1fa9-7249-a1ad-01b39d917651" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-7249-a1ad-01b39d917651_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-7b6a-8712-83183a1adb88" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1fa9-7249-a1ad-01b39d917651" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-7b6a-8712-83183a1adb88" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_PublicStockOfferingMember_019ff692-1fa9-78d2-9912-107a9f64d781" xlink:href="mlys-20260930.xsd#mlys_PublicStockOfferingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-7b6a-8712-83183a1adb88" xlink:to="loc_mlys_PublicStockOfferingMember_019ff692-1fa9-78d2-9912-107a9f64d781" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_AtTheMarketAgreementMember_019ff692-1fa9-78e4-827f-ca9dcbd467e8" xlink:href="mlys-20260930.xsd#mlys_AtTheMarketAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-7b6a-8712-83183a1adb88" xlink:to="loc_mlys_AtTheMarketAgreementMember_019ff692-1fa9-78e4-827f-ca9dcbd467e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_019ff692-1fa9-7cfa-8c40-3f9d4cb68135" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ff692-1fa9-71ea-8d3f-f3682f1e969c" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_019ff692-1fa9-7cfa-8c40-3f9d4cb68135" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/CondensedStatementsofCashFlows" xlink:type="simple" xlink:href="mlys-20260930.xsd#CondensedStatementsofCashFlows"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/CondensedStatementsofCashFlows" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019ff692-1fa9-7cd3-94cd-ce8fc38de9bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019ff692-1fa9-7efb-835b-6d0f259372af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019ff692-1fa9-7cd3-94cd-ce8fc38de9bd" xlink:to="loc_us-gaap_StatementTable_019ff692-1fa9-7efb-835b-6d0f259372af" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1fa9-74fd-8819-7771a3937d5d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ff692-1fa9-7efb-835b-6d0f259372af" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1fa9-74fd-8819-7771a3937d5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-74fd-8819-7771a3937d5d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1fa9-74fd-8819-7771a3937d5d" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-74fd-8819-7771a3937d5d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-71ab-b45d-cc1fc802e3c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1fa9-74fd-8819-7771a3937d5d" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-71ab-b45d-cc1fc802e3c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_PublicOfferingSeptember2025Member_019ff692-1fa9-7e5c-9bac-63b4da1fc5e1" xlink:href="mlys-20260930.xsd#mlys_PublicOfferingSeptember2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-71ab-b45d-cc1fc802e3c7" xlink:to="loc_mlys_PublicOfferingSeptember2025Member_019ff692-1fa9-7e5c-9bac-63b4da1fc5e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_AtTheMarketAgreementMember_019ff692-1fa9-7b97-8dfc-1d3afdbb7dec" xlink:href="mlys-20260930.xsd#mlys_AtTheMarketAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1fa9-71ab-b45d-cc1fc802e3c7" xlink:to="loc_mlys_AtTheMarketAgreementMember_019ff692-1fa9-7b97-8dfc-1d3afdbb7dec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019ff692-1fa9-76e2-a651-f6bbc046e858" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ff692-1fa9-7cd3-94cd-ce8fc38de9bd" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019ff692-1fa9-76e2-a651-f6bbc046e858" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019ff692-1fa9-7a57-a1e0-57348dd898f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019ff692-1fa9-76e2-a651-f6bbc046e858" xlink:to="loc_us-gaap_NetIncomeLoss_019ff692-1fa9-7a57-a1e0-57348dd898f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019ff692-1fa9-730a-950a-89c0e64baa40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019ff692-1fa9-76e2-a651-f6bbc046e858" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019ff692-1fa9-730a-950a-89c0e64baa40" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_LicenseAgreementUpfrontCashPaymentOperatingActivities_019ff692-1fa9-73a2-a6ed-086966e8e7d7" xlink:href="mlys-20260930.xsd#mlys_LicenseAgreementUpfrontCashPaymentOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019ff692-1fa9-730a-950a-89c0e64baa40" xlink:to="loc_mlys_LicenseAgreementUpfrontCashPaymentOperatingActivities_019ff692-1fa9-73a2-a6ed-086966e8e7d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_019ff692-1fa9-7273-a249-b3c42fe0ff63" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019ff692-1fa9-730a-950a-89c0e64baa40" xlink:to="loc_us-gaap_ShareBasedCompensation_019ff692-1fa9-7273-a249-b3c42fe0ff63" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments_019ff692-1fa9-7d30-a588-c5b66295833e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019ff692-1fa9-730a-950a-89c0e64baa40" xlink:to="loc_us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments_019ff692-1fa9-7d30-a588-c5b66295833e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCostsAndDiscounts_019ff692-1fa9-740d-bdc3-50099cac6503" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfFinancingCostsAndDiscounts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019ff692-1fa9-730a-950a-89c0e64baa40" xlink:to="loc_us-gaap_AmortizationOfFinancingCostsAndDiscounts_019ff692-1fa9-740d-bdc3-50099cac6503" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_019ff692-1fa9-7fab-ae43-3059c51a4bd4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019ff692-1fa9-730a-950a-89c0e64baa40" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_019ff692-1fa9-7fab-ae43-3059c51a4bd4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingAssetsAbstract_019ff692-1fa9-7e0a-bc67-42684a6521e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOperatingAssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019ff692-1fa9-730a-950a-89c0e64baa40" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingAssetsAbstract_019ff692-1fa9-7e0a-bc67-42684a6521e9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet_019ff692-1fa9-7796-8216-21785d8dc607" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingAssetsAbstract_019ff692-1fa9-7e0a-bc67-42684a6521e9" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet_019ff692-1fa9-7796-8216-21785d8dc607" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_019ff692-1fa9-7f44-9a77-95d702408b01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingAssetsAbstract_019ff692-1fa9-7e0a-bc67-42684a6521e9" xlink:to="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_019ff692-1fa9-7f44-9a77-95d702408b01" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities_019ff692-1fa9-7cd3-83b9-45f40f19870a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingAssetsAbstract_019ff692-1fa9-7e0a-bc67-42684a6521e9" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities_019ff692-1fa9-7cd3-83b9-45f40f19870a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019ff692-1fa9-7915-878c-4ce15de72ab5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019ff692-1fa9-76e2-a651-f6bbc046e858" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019ff692-1fa9-7915-878c-4ce15de72ab5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019ff692-1fa9-72cf-baa1-f59905dc9964" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ff692-1fa9-7cd3-94cd-ce8fc38de9bd" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019ff692-1fa9-72cf-baa1-f59905dc9964" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireHeldToMaturitySecurities_019ff692-1fa9-7687-9576-d2f1d04dcad4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquireHeldToMaturitySecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019ff692-1fa9-72cf-baa1-f59905dc9964" xlink:to="loc_us-gaap_PaymentsToAcquireHeldToMaturitySecurities_019ff692-1fa9-7687-9576-d2f1d04dcad4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfHeldToMaturitySecurities_019ff692-1fa9-7be0-a5a6-c26c25f3d95a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfHeldToMaturitySecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019ff692-1fa9-72cf-baa1-f59905dc9964" xlink:to="loc_us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfHeldToMaturitySecurities_019ff692-1fa9-7be0-a5a6-c26c25f3d95a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_LicenseAgreementUpfrontCashPaymentInvestingActivities_019ff692-1fa9-73c1-bf88-71f08c4f5304" xlink:href="mlys-20260930.xsd#mlys_LicenseAgreementUpfrontCashPaymentInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019ff692-1fa9-72cf-baa1-f59905dc9964" xlink:to="loc_mlys_LicenseAgreementUpfrontCashPaymentInvestingActivities_019ff692-1fa9-73c1-bf88-71f08c4f5304" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForProceedsFromOtherDeposits_01a13d3e-da75-7468-8515-4b9ca2105b62" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsForProceedsFromOtherDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019ff692-1fa9-72cf-baa1-f59905dc9964" xlink:to="loc_us-gaap_PaymentsForProceedsFromOtherDeposits_01a13d3e-da75-7468-8515-4b9ca2105b62" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019ff692-1fa9-75f6-8433-9a0f851b2907" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019ff692-1fa9-72cf-baa1-f59905dc9964" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019ff692-1fa9-75f6-8433-9a0f851b2907" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019ff692-1fa9-7a8c-a59e-9ee26d08e999" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ff692-1fa9-7cd3-94cd-ce8fc38de9bd" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019ff692-1fa9-7a8c-a59e-9ee26d08e999" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_019ff692-1fa9-7805-86f0-7d12e36e5350" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019ff692-1fa9-7a8c-a59e-9ee26d08e999" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_019ff692-1fa9-7805-86f0-7d12e36e5350" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt_019ff692-1fa9-7cdf-83a4-acbe8699073e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019ff692-1fa9-7a8c-a59e-9ee26d08e999" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt_019ff692-1fa9-7cdf-83a4-acbe8699073e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDebtIssuanceCosts_019ff692-1fa9-7eff-ba6e-d4634bbc17ef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsOfDebtIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019ff692-1fa9-7a8c-a59e-9ee26d08e999" xlink:to="loc_us-gaap_PaymentsOfDebtIssuanceCosts_019ff692-1fa9-7eff-ba6e-d4634bbc17ef" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromStockOptionsExercised_019ff692-1fa9-7dfd-87d8-33a1d8f1ef08" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019ff692-1fa9-7a8c-a59e-9ee26d08e999" xlink:to="loc_us-gaap_ProceedsFromStockOptionsExercised_019ff692-1fa9-7dfd-87d8-33a1d8f1ef08" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromStockPlans_019ff692-1fa9-710e-a032-2218fa387cbd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromStockPlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019ff692-1fa9-7a8c-a59e-9ee26d08e999" xlink:to="loc_us-gaap_ProceedsFromStockPlans_019ff692-1fa9-710e-a032-2218fa387cbd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfStockIssuanceCosts_019ff692-1fa9-718c-83f2-af8ad089ed4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsOfStockIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019ff692-1fa9-7a8c-a59e-9ee26d08e999" xlink:to="loc_us-gaap_PaymentsOfStockIssuanceCosts_019ff692-1fa9-718c-83f2-af8ad089ed4a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019ff692-1fa9-7814-8f63-4b6cf7888b6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019ff692-1fa9-7a8c-a59e-9ee26d08e999" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019ff692-1fa9-7814-8f63-4b6cf7888b6d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_019ff692-1fa9-733c-9113-7b0fb8f8910d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ff692-1fa9-7cd3-94cd-ce8fc38de9bd" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_019ff692-1fa9-733c-9113-7b0fb8f8910d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019ff692-1fa9-749a-bb1a-0e42e2afd0a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ff692-1fa9-7cd3-94cd-ce8fc38de9bd" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019ff692-1fa9-749a-bb1a-0e42e2afd0a3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019ff692-1fa9-7d70-8bde-63e5f4255be4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplementalCashFlowElementsAbstract_019ff692-1fa9-7904-b9cd-df3447a6b522" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SupplementalCashFlowElementsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ff692-1fa9-7cd3-94cd-ce8fc38de9bd" xlink:to="loc_us-gaap_SupplementalCashFlowElementsAbstract_019ff692-1fa9-7904-b9cd-df3447a6b522" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_NoncashOrPartNoncashTransactionAccretionOfExitConsiderationOnSeniorLongTermDebt_019ff692-1fa9-73f8-a0c1-38cfabe11bd8" xlink:href="mlys-20260930.xsd#mlys_NoncashOrPartNoncashTransactionAccretionOfExitConsiderationOnSeniorLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowElementsAbstract_019ff692-1fa9-7904-b9cd-df3447a6b522" xlink:to="loc_mlys_NoncashOrPartNoncashTransactionAccretionOfExitConsiderationOnSeniorLongTermDebt_019ff692-1fa9-73f8-a0c1-38cfabe11bd8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_NoncashOrPartNoncashTransactionOfferingCosts_019ff692-1fa9-7925-b52c-be58697f4aac" xlink:href="mlys-20260930.xsd#mlys_NoncashOrPartNoncashTransactionOfferingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowElementsAbstract_019ff692-1fa9-7904-b9cd-df3447a6b522" xlink:to="loc_mlys_NoncashOrPartNoncashTransactionOfferingCosts_019ff692-1fa9-7925-b52c-be58697f4aac" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/SummaryofSignificantAccountingPoliciesScheduleofPotentialCommonSharesExcludedFromCalculationofNetLossPerShareDetails" xlink:type="simple" xlink:href="mlys-20260930.xsd#SummaryofSignificantAccountingPoliciesScheduleofPotentialCommonSharesExcludedFromCalculationofNetLossPerShareDetails"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/SummaryofSignificantAccountingPoliciesScheduleofPotentialCommonSharesExcludedFromCalculationofNetLossPerShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-1faa-733c-b256-a27fbc062666" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-1faa-7d55-92ff-219a12a370bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-1faa-733c-b256-a27fbc062666" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-1faa-7d55-92ff-219a12a370bd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019ff692-1faa-71a8-9304-671fae777f1c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-1faa-7d55-92ff-219a12a370bd" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019ff692-1faa-71a8-9304-671fae777f1c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019ff692-1faa-71a8-9304-671fae777f1c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019ff692-1faa-71a8-9304-671fae777f1c" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019ff692-1faa-71a8-9304-671fae777f1c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019ff692-1faa-7fb1-adc9-30562af84dfe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019ff692-1faa-71a8-9304-671fae777f1c" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019ff692-1faa-7fb1-adc9-30562af84dfe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019ff692-1faa-7fa6-a580-1b94ffb8afbf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019ff692-1faa-7fb1-adc9-30562af84dfe" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019ff692-1faa-7fa6-a580-1b94ffb8afbf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019ff692-1faa-76b1-a013-22b7d5c62b72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019ff692-1faa-7fb1-adc9-30562af84dfe" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019ff692-1faa-76b1-a013-22b7d5c62b72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019ff692-1faa-7164-afcf-635b6127cce1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019ff692-1faa-7fb1-adc9-30562af84dfe" xlink:to="loc_us-gaap_RestrictedStockMember_019ff692-1faa-7164-afcf-635b6127cce1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019ff692-1faa-7460-889f-02a0b7d387e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-1faa-733c-b256-a27fbc062666" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019ff692-1faa-7460-889f-02a0b7d387e7" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/FairValueofFinancialInstrumentsScheduleofFinancialInstrumentsMeasuredatFairValueHierarchyDetails" xlink:type="simple" xlink:href="mlys-20260930.xsd#FairValueofFinancialInstrumentsScheduleofFinancialInstrumentsMeasuredatFairValueHierarchyDetails"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/FairValueofFinancialInstrumentsScheduleofFinancialInstrumentsMeasuredatFairValueHierarchyDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsLineItems_019ff692-1faa-7338-9200-adf14449d1ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019ff692-1faa-783f-ade5-a09549615ff1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019ff692-1faa-7338-9200-adf14449d1ce" xlink:to="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019ff692-1faa-783f-ade5-a09549615ff1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_019ff692-1faa-799f-8d78-5020960da8aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019ff692-1faa-783f-ade5-a09549615ff1" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_019ff692-1faa-799f-8d78-5020960da8aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019ff692-1faa-799f-8d78-5020960da8aa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019ff692-1faa-799f-8d78-5020960da8aa" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019ff692-1faa-799f-8d78-5020960da8aa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019ff692-1faa-7712-bffa-5c76aa6e2272" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019ff692-1faa-799f-8d78-5020960da8aa" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019ff692-1faa-7712-bffa-5c76aa6e2272" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_019ff692-1faa-788f-8edc-72e6125361e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019ff692-1faa-7712-bffa-5c76aa6e2272" xlink:to="loc_us-gaap_MoneyMarketFundsMember_019ff692-1faa-788f-8edc-72e6125361e9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ff692-1faa-7996-b15d-108d87207619" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019ff692-1faa-783f-ade5-a09549615ff1" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ff692-1faa-7996-b15d-108d87207619" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ff692-1faa-7996-b15d-108d87207619_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ff692-1faa-7996-b15d-108d87207619" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ff692-1faa-7996-b15d-108d87207619_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ff692-1faa-7ed3-b62e-48c769e20558" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ff692-1faa-7996-b15d-108d87207619" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ff692-1faa-7ed3-b62e-48c769e20558" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019ff692-1faa-795e-aa27-7f97b3c2cd9f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ff692-1faa-7ed3-b62e-48c769e20558" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019ff692-1faa-795e-aa27-7f97b3c2cd9f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019ff692-1faa-7fcd-9626-cddc710d33d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019ff692-1faa-7338-9200-adf14449d1ce" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019ff692-1faa-7fcd-9626-cddc710d33d9" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/FairValueofFinancialInstrumentsScheduleofHeldtoMaturityUSTreasuryBillsDetails" xlink:type="simple" xlink:href="mlys-20260930.xsd#FairValueofFinancialInstrumentsScheduleofHeldtoMaturityUSTreasuryBillsDetails"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/FairValueofFinancialInstrumentsScheduleofHeldtoMaturityUSTreasuryBillsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsLineItems_019ff692-1faa-7ecd-94d7-245a1dbf3d67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019ff692-1faa-76c7-b20a-bc4e4f134866" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019ff692-1faa-7ecd-94d7-245a1dbf3d67" xlink:to="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019ff692-1faa-76c7-b20a-bc4e4f134866" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfFinancialPositionLocationBalanceAxis_019ff692-1faa-7237-ab3b-2d5f0cb7b646" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfFinancialPositionLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019ff692-1faa-76c7-b20a-bc4e4f134866" xlink:to="loc_us-gaap_StatementOfFinancialPositionLocationBalanceAxis_019ff692-1faa-7237-ab3b-2d5f0cb7b646" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract_019ff692-1faa-7503-a28b-4634f7a555dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019ff692-1faa-7ecd-94d7-245a1dbf3d67" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract_019ff692-1faa-7503-a28b-4634f7a555dc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLossCurrent_019ff692-1faa-784e-984f-39673af31d49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract_019ff692-1faa-7503-a28b-4634f7a555dc" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLossCurrent_019ff692-1faa-784e-984f-39673af31d49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss_019ff692-1faa-77bc-8930-0ab764c89f97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract_019ff692-1faa-7503-a28b-4634f7a555dc" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss_019ff692-1faa-77bc-8930-0ab764c89f97" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityFairValueAbstract_019ff692-1faa-7db7-9d09-5a6089548c79" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesHeldToMaturityFairValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019ff692-1faa-7ecd-94d7-245a1dbf3d67" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityFairValueAbstract_019ff692-1faa-7db7-9d09-5a6089548c79" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityFairValueCurrent_019ff692-1faa-7782-be2f-e3f631fd3048" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesHeldToMaturityFairValueCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesHeldToMaturityFairValueAbstract_019ff692-1faa-7db7-9d09-5a6089548c79" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityFairValueCurrent_019ff692-1faa-7782-be2f-e3f631fd3048" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019ff692-1faa-7d99-89b7-3e93714fb4e6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesHeldToMaturityFairValueAbstract_019ff692-1faa-7db7-9d09-5a6089548c79" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019ff692-1faa-7d99-89b7-3e93714fb4e6" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/FairValueofFinancialInstrumentsScheduleofHeldtoMaturityUSTreasuryBillsDetails_1" xlink:type="simple" xlink:href="mlys-20260930.xsd#FairValueofFinancialInstrumentsScheduleofHeldtoMaturityUSTreasuryBillsDetails_1"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/FairValueofFinancialInstrumentsScheduleofHeldtoMaturityUSTreasuryBillsDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsLineItems_019ff692-1faa-7ecd-94d7-245a1dbf3d67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019ff692-1faa-76c7-b20a-bc4e4f134866" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019ff692-1faa-7ecd-94d7-245a1dbf3d67" xlink:to="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019ff692-1faa-76c7-b20a-bc4e4f134866" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract_019ff692-1faa-7503-a28b-4634f7a555dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019ff692-1faa-7ecd-94d7-245a1dbf3d67" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract_019ff692-1faa-7503-a28b-4634f7a555dc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLossCurrent_019ff692-1faa-784e-984f-39673af31d49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract_019ff692-1faa-7503-a28b-4634f7a555dc" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLossCurrent_019ff692-1faa-784e-984f-39673af31d49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss_019ff692-1faa-77bc-8930-0ab764c89f97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesNetCarryingAmountAbstract_019ff692-1faa-7503-a28b-4634f7a555dc" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostAfterAllowanceForCreditLoss_019ff692-1faa-77bc-8930-0ab764c89f97" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityFairValueAbstract_019ff692-1faa-7db7-9d09-5a6089548c79" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesHeldToMaturityFairValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019ff692-1faa-7ecd-94d7-245a1dbf3d67" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityFairValueAbstract_019ff692-1faa-7db7-9d09-5a6089548c79" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityFairValueCurrent_019ff692-1faa-7782-be2f-e3f631fd3048" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesHeldToMaturityFairValueCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesHeldToMaturityFairValueAbstract_019ff692-1faa-7db7-9d09-5a6089548c79" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityFairValueCurrent_019ff692-1faa-7782-be2f-e3f631fd3048" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019ff692-1faa-7d99-89b7-3e93714fb4e6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesHeldToMaturityFairValueAbstract_019ff692-1faa-7db7-9d09-5a6089548c79" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019ff692-1faa-7d99-89b7-3e93714fb4e6" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/CommitmentsandContingenciesDetails" xlink:type="simple" xlink:href="mlys-20260930.xsd#CommitmentsandContingenciesDetails"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/CommitmentsandContingenciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsLineItems_019ff692-1faa-7a8b-9229-b1083fec35cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsTable_019ff692-1faa-7d49-8304-c00c3e653e61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019ff692-1faa-7a8b-9229-b1083fec35cd" xlink:to="loc_us-gaap_OtherCommitmentsTable_019ff692-1faa-7d49-8304-c00c3e653e61" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019ff692-1faa-7458-97b4-15c7b0cd71f0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_019ff692-1faa-7d49-8304-c00c3e653e61" xlink:to="loc_srt_CounterpartyNameAxis_019ff692-1faa-7458-97b4-15c7b0cd71f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ff692-1faa-7458-97b4-15c7b0cd71f0_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019ff692-1faa-7458-97b4-15c7b0cd71f0" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ff692-1faa-7458-97b4-15c7b0cd71f0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ff692-1faa-7352-9092-645dada5b58a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019ff692-1faa-7458-97b4-15c7b0cd71f0" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ff692-1faa-7352-9092-645dada5b58a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_TanabeMember_019ff692-1faa-7139-9d6a-a67d950e31c6" xlink:href="mlys-20260930.xsd#mlys_TanabeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ff692-1faa-7352-9092-645dada5b58a" xlink:to="loc_mlys_TanabeMember_019ff692-1faa-7139-9d6a-a67d950e31c6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfArrangementAxis_019ff692-1faa-73fd-ba92-d2888b4430a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_019ff692-1faa-7d49-8304-c00c3e653e61" xlink:to="loc_us-gaap_TypeOfArrangementAxis_019ff692-1faa-73fd-ba92-d2888b4430a1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019ff692-1faa-73fd-ba92-d2888b4430a1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_019ff692-1faa-73fd-ba92-d2888b4430a1" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019ff692-1faa-73fd-ba92-d2888b4430a1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019ff692-1faa-7b90-acb5-81d54a865f74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_019ff692-1faa-73fd-ba92-d2888b4430a1" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019ff692-1faa-7b90-acb5-81d54a865f74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember_019ff692-1faa-7df3-a3f5-bf18fb6f79a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019ff692-1faa-7b90-acb5-81d54a865f74" xlink:to="loc_us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember_019ff692-1faa-7df3-a3f5-bf18fb6f79a4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_CollaborativeArrangementRightsAndObligationsUpfrontFee_019ff692-1faa-7770-bf4e-7629189cad6e" xlink:href="mlys-20260930.xsd#mlys_CollaborativeArrangementRightsAndObligationsUpfrontFee"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019ff692-1faa-7a8b-9229-b1083fec35cd" xlink:to="loc_mlys_CollaborativeArrangementRightsAndObligationsUpfrontFee_019ff692-1faa-7770-bf4e-7629189cad6e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_CollaborativeArrangementRightsAndObligationsDevelopmentalMaximumAggregateMilestonePayments_019ff692-1faa-78e7-97b9-732ed1537b18" xlink:href="mlys-20260930.xsd#mlys_CollaborativeArrangementRightsAndObligationsDevelopmentalMaximumAggregateMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019ff692-1faa-7a8b-9229-b1083fec35cd" xlink:to="loc_mlys_CollaborativeArrangementRightsAndObligationsDevelopmentalMaximumAggregateMilestonePayments_019ff692-1faa-78e7-97b9-732ed1537b18" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_CollaborativeArrangementRightsAndObligationsCommercialSaleMaximumAggregateMilestoneOneTimePayments_019ff692-1faa-7588-bfd4-00f81bd90d9c" xlink:href="mlys-20260930.xsd#mlys_CollaborativeArrangementRightsAndObligationsCommercialSaleMaximumAggregateMilestoneOneTimePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019ff692-1faa-7a8b-9229-b1083fec35cd" xlink:to="loc_mlys_CollaborativeArrangementRightsAndObligationsCommercialSaleMaximumAggregateMilestoneOneTimePayments_019ff692-1faa-7588-bfd4-00f81bd90d9c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_CollaborativeArrangementRightsAndObligationsAdditionalCommercialSaleMaximumAggregateMilestonePayments_019ff692-1faa-7a9e-9884-f84f8678c62f" xlink:href="mlys-20260930.xsd#mlys_CollaborativeArrangementRightsAndObligationsAdditionalCommercialSaleMaximumAggregateMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019ff692-1faa-7a8b-9229-b1083fec35cd" xlink:to="loc_mlys_CollaborativeArrangementRightsAndObligationsAdditionalCommercialSaleMaximumAggregateMilestonePayments_019ff692-1faa-7a9e-9884-f84f8678c62f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_CollaborativeArrangementRightsAndObligationsFirstCommercialSaleMaximumAggregateMilestonePayments_019ff692-1faa-7383-92c0-77ae0d83612e" xlink:href="mlys-20260930.xsd#mlys_CollaborativeArrangementRightsAndObligationsFirstCommercialSaleMaximumAggregateMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019ff692-1faa-7a8b-9229-b1083fec35cd" xlink:to="loc_mlys_CollaborativeArrangementRightsAndObligationsFirstCommercialSaleMaximumAggregateMilestonePayments_019ff692-1faa-7383-92c0-77ae0d83612e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_CollaborativeArrangementRightsAndObligationsSecondCommercialSaleMaximumAggregateMilestonePayments_019ff692-1faa-73b5-88b9-54327c4c5115" xlink:href="mlys-20260930.xsd#mlys_CollaborativeArrangementRightsAndObligationsSecondCommercialSaleMaximumAggregateMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019ff692-1faa-7a8b-9229-b1083fec35cd" xlink:to="loc_mlys_CollaborativeArrangementRightsAndObligationsSecondCommercialSaleMaximumAggregateMilestonePayments_019ff692-1faa-73b5-88b9-54327c4c5115" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_CollaborativeArrangementRightsAndObligationsRecognizedOneTimePayments_019ff692-1faa-78ca-bfd0-b04225233ae2" xlink:href="mlys-20260930.xsd#mlys_CollaborativeArrangementRightsAndObligationsRecognizedOneTimePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019ff692-1faa-7a8b-9229-b1083fec35cd" xlink:to="loc_mlys_CollaborativeArrangementRightsAndObligationsRecognizedOneTimePayments_019ff692-1faa-78ca-bfd0-b04225233ae2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_CollaborativeArrangementRightsAndObligationsFirstCommercialSaleMilestonePayments_019ff692-1faa-790b-820a-01582c369ffd" xlink:href="mlys-20260930.xsd#mlys_CollaborativeArrangementRightsAndObligationsFirstCommercialSaleMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019ff692-1faa-7a8b-9229-b1083fec35cd" xlink:to="loc_mlys_CollaborativeArrangementRightsAndObligationsFirstCommercialSaleMilestonePayments_019ff692-1faa-790b-820a-01582c369ffd" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AreaOfRealEstateProperty_01a13d5d-3c2e-707e-8553-ba6efeacd304" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AreaOfRealEstateProperty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019ff692-1faa-7a8b-9229-b1083fec35cd" xlink:to="loc_us-gaap_AreaOfRealEstateProperty_01a13d5d-3c2e-707e-8553-ba6efeacd304" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_01a13d5d-3c2e-7a0d-871f-0e72c0800bc8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019ff692-1faa-7a8b-9229-b1083fec35cd" xlink:to="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_01a13d5d-3c2e-7a0d-871f-0e72c0800bc8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount_01a15799-52e4-7ab4-8cd6-dd252c2be0b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019ff692-1faa-7a8b-9229-b1083fec35cd" xlink:to="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount_01a15799-52e4-7ab4-8cd6-dd252c2be0b3" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/SeniorSecuredTermLoanNarrativeDetails" xlink:type="simple" xlink:href="mlys-20260930.xsd#SeniorSecuredTermLoanNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/SeniorSecuredTermLoanNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_019ff692-1faa-7dd6-b68e-0e56ac4e2d49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_019ff692-1faa-7dd6-b68e-0e56ac4e2d49" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019ff692-1faa-742d-b936-35a40f1eba16" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019ff692-1faa-7dd6-b68e-0e56ac4e2d49" xlink:to="loc_us-gaap_CreditFacilityAxis_019ff692-1faa-742d-b936-35a40f1eba16" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019ff692-1faa-742d-b936-35a40f1eba16_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019ff692-1faa-742d-b936-35a40f1eba16" xlink:to="loc_us-gaap_CreditFacilityDomain_019ff692-1faa-742d-b936-35a40f1eba16_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019ff692-1faa-785e-ab8b-4651331e878d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019ff692-1faa-742d-b936-35a40f1eba16" xlink:to="loc_us-gaap_CreditFacilityDomain_019ff692-1faa-785e-ab8b-4651331e878d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019ff692-1faa-7685-8602-6ac8aae124c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019ff692-1faa-785e-ab8b-4651331e878d" xlink:to="loc_us-gaap_SecuredDebtMember_019ff692-1faa-7685-8602-6ac8aae124c9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019ff692-1faa-7fea-9399-a5fd1e7a1400" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019ff692-1faa-7dd6-b68e-0e56ac4e2d49" xlink:to="loc_us-gaap_DebtInstrumentAxis_019ff692-1faa-7fea-9399-a5fd1e7a1400" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ff692-1faa-7fea-9399-a5fd1e7a1400_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ff692-1faa-7fea-9399-a5fd1e7a1400" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ff692-1faa-7fea-9399-a5fd1e7a1400_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ff692-1faa-71ba-a6e7-2d60fff1026c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ff692-1faa-7fea-9399-a5fd1e7a1400" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ff692-1faa-71ba-a6e7-2d60fff1026c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_TheLoanAgreementMember_019ff692-1faa-782e-ab7a-4b41e72cee84" xlink:href="mlys-20260930.xsd#mlys_TheLoanAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ff692-1faa-71ba-a6e7-2d60fff1026c" xlink:to="loc_mlys_TheLoanAgreementMember_019ff692-1faa-782e-ab7a-4b41e72cee84" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019ff692-1faa-7dda-a36c-33b176a5f031" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019ff692-1faa-7dd6-b68e-0e56ac4e2d49" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019ff692-1faa-7dda-a36c-33b176a5f031" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ff692-1faa-7dda-a36c-33b176a5f031_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ff692-1faa-7dda-a36c-33b176a5f031" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ff692-1faa-7dda-a36c-33b176a5f031_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ff692-1faa-7ed3-8e58-d22f7d5c8047" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ff692-1faa-7dda-a36c-33b176a5f031" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ff692-1faa-7ed3-8e58-d22f7d5c8047" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019ff692-1faa-7a2b-b84b-2b0111e014e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ff692-1faa-7ed3-8e58-d22f7d5c8047" xlink:to="loc_us-gaap_LineOfCreditMember_019ff692-1faa-7a2b-b84b-2b0111e014e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_DebtInstrumentTranchesAxis_019ff692-1faa-7e90-abf0-cf9d932c272c" xlink:href="mlys-20260930.xsd#mlys_DebtInstrumentTranchesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019ff692-1faa-7dd6-b68e-0e56ac4e2d49" xlink:to="loc_mlys_DebtInstrumentTranchesAxis_019ff692-1faa-7e90-abf0-cf9d932c272c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_DebtInstrumentTranchesDomain_019ff692-1faa-7e90-abf0-cf9d932c272c_default" xlink:href="mlys-20260930.xsd#mlys_DebtInstrumentTranchesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_mlys_DebtInstrumentTranchesAxis_019ff692-1faa-7e90-abf0-cf9d932c272c" xlink:to="loc_mlys_DebtInstrumentTranchesDomain_019ff692-1faa-7e90-abf0-cf9d932c272c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_DebtInstrumentTranchesDomain_019ff692-1faa-7b85-b906-39f911f5043c" xlink:href="mlys-20260930.xsd#mlys_DebtInstrumentTranchesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_mlys_DebtInstrumentTranchesAxis_019ff692-1faa-7e90-abf0-cf9d932c272c" xlink:to="loc_mlys_DebtInstrumentTranchesDomain_019ff692-1faa-7b85-b906-39f911f5043c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_TrancheALoanMember_019ff692-1faa-7cf8-86bb-a1f3657bba7a" xlink:href="mlys-20260930.xsd#mlys_TrancheALoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mlys_DebtInstrumentTranchesDomain_019ff692-1faa-7b85-b906-39f911f5043c" xlink:to="loc_mlys_TrancheALoanMember_019ff692-1faa-7cf8-86bb-a1f3657bba7a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_TrancheBLoanMember_019ff692-1faa-7ae6-ad21-942cb59d4f9a" xlink:href="mlys-20260930.xsd#mlys_TrancheBLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mlys_DebtInstrumentTranchesDomain_019ff692-1faa-7b85-b906-39f911f5043c" xlink:to="loc_mlys_TrancheBLoanMember_019ff692-1faa-7ae6-ad21-942cb59d4f9a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_TrancheCLoanMember_019ff692-1faa-7cff-8174-4bdfebbf3640" xlink:href="mlys-20260930.xsd#mlys_TrancheCLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mlys_DebtInstrumentTranchesDomain_019ff692-1faa-7b85-b906-39f911f5043c" xlink:to="loc_mlys_TrancheCLoanMember_019ff692-1faa-7cff-8174-4bdfebbf3640" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_TrancheDLoanMember_019ff692-1faa-7237-8b65-24c5c22b357e" xlink:href="mlys-20260930.xsd#mlys_TrancheDLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mlys_DebtInstrumentTranchesDomain_019ff692-1faa-7b85-b906-39f911f5043c" xlink:to="loc_mlys_TrancheDLoanMember_019ff692-1faa-7237-8b65-24c5c22b357e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_019ff692-1faa-74bd-8651-70eadd63cfd0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_us-gaap_DebtInstrumentTerm_019ff692-1faa-74bd-8651-70eadd63cfd0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromLongTermLinesOfCredit_019ff692-1faa-71fe-b159-2261bdf1e717" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromLongTermLinesOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_us-gaap_ProceedsFromLongTermLinesOfCredit_019ff692-1faa-71fe-b159-2261bdf1e717" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_DebtInstrumentBasisFloor_019ff692-1faa-78a7-8b0a-5aa696be8ec7" xlink:href="mlys-20260930.xsd#mlys_DebtInstrumentBasisFloor"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_mlys_DebtInstrumentBasisFloor_019ff692-1faa-78a7-8b0a-5aa696be8ec7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ff692-1faa-756f-abf7-48125c4e8b54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ff692-1faa-756f-abf7-48125c4e8b54" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019ff692-1faa-7f0b-8055-b1eb5938ffbe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019ff692-1faa-7f0b-8055-b1eb5938ffbe" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_DebtInstrumentFundingFeePercentage_019ff692-1faa-7843-9c3e-423b58def9a8" xlink:href="mlys-20260930.xsd#mlys_DebtInstrumentFundingFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_mlys_DebtInstrumentFundingFeePercentage_019ff692-1faa-7843-9c3e-423b58def9a8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_DebtInstrumentFundingFeePaid_019ff692-1faa-7608-b44c-c8acd2152c32" xlink:href="mlys-20260930.xsd#mlys_DebtInstrumentFundingFeePaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_mlys_DebtInstrumentFundingFeePaid_019ff692-1faa-7608-b44c-c8acd2152c32" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_DebtInstrumentInstallmentsLoanRepaymentTerm_019ff692-1faa-799d-abc4-e2c0ea492768" xlink:href="mlys-20260930.xsd#mlys_DebtInstrumentInstallmentsLoanRepaymentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_mlys_DebtInstrumentInstallmentsLoanRepaymentTerm_019ff692-1faa-799d-abc4-e2c0ea492768" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_DebtInstrumentFundingFeeFinalFeeAsAPercentageOfOriginalPrincipal_019ff692-1faa-79ce-8d97-163ae1938f11" xlink:href="mlys-20260930.xsd#mlys_DebtInstrumentFundingFeeFinalFeeAsAPercentageOfOriginalPrincipal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_mlys_DebtInstrumentFundingFeeFinalFeeAsAPercentageOfOriginalPrincipal_019ff692-1faa-79ce-8d97-163ae1938f11" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_DebtInstrumentFundingFeeFinalLoanRepayment_019ff692-1faa-7ddb-88fa-d372d1b76ba5" xlink:href="mlys-20260930.xsd#mlys_DebtInstrumentFundingFeeFinalLoanRepayment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_mlys_DebtInstrumentFundingFeeFinalLoanRepayment_019ff692-1faa-7ddb-88fa-d372d1b76ba5" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_DebtInstrumentCovenantMinimumTrailingConsolidatedNetProductRevenuePeriod_019ff692-1faa-7eaa-921a-e72f21c87d9c" xlink:href="mlys-20260930.xsd#mlys_DebtInstrumentCovenantMinimumTrailingConsolidatedNetProductRevenuePeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_mlys_DebtInstrumentCovenantMinimumTrailingConsolidatedNetProductRevenuePeriod_019ff692-1faa-7eaa-921a-e72f21c87d9c" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromLinesOfCredit_019ff692-1faa-724c-a275-627460e8b994" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromLinesOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_us-gaap_ProceedsFromLinesOfCredit_019ff692-1faa-724c-a275-627460e8b994" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019ff692-1faa-7d38-bde4-d08d02cf4ce0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019ff692-1faa-7d38-bde4-d08d02cf4ce0" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_019ff692-1faa-7230-bdc0-d9144d4c4566" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_019ff692-1faa-7230-bdc0-d9144d4c4566" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNoncurrentGross_01a1a7d8-4a1b-7c91-99a8-25d6613e5ace" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredFinanceCostsNoncurrentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_us-gaap_DeferredFinanceCostsNoncurrentGross_01a1a7d8-4a1b-7c91-99a8-25d6613e5ace" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019ff692-1faa-7a63-a8b0-774d0e65bc59" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019ff692-1faa-7a63-a8b0-774d0e65bc59" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_019ff692-1faa-7a20-b5f0-56c6c98f9459" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_us-gaap_InterestExpenseDebt_019ff692-1faa-7a20-b5f0-56c6c98f9459" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_DebtInstrumentNumberOfQuarterlyPayments_01a19cd9-2227-76d4-92c7-e41ccd28dde7" xlink:href="mlys-20260930.xsd#mlys_DebtInstrumentNumberOfQuarterlyPayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_mlys_DebtInstrumentNumberOfQuarterlyPayments_01a19cd9-2227-76d4-92c7-e41ccd28dde7" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentPeriodicPaymentPrincipal_019ff692-1faa-796b-a0d2-aebea358414d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentPeriodicPaymentPrincipal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_us-gaap_DebtInstrumentPeriodicPaymentPrincipal_019ff692-1faa-796b-a0d2-aebea358414d" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseLongTermDebt_01a157b7-b4d5-767c-a7ca-f58166335013" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-7660-bb44-117472444f82" xlink:to="loc_us-gaap_InterestExpenseLongTermDebt_01a157b7-b4d5-767c-a7ca-f58166335013" xlink:type="arc" order="19"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/SeniorSecuredTermLoanScheduleofTermLoanDetails" xlink:type="simple" xlink:href="mlys-20260930.xsd#SeniorSecuredTermLoanScheduleofTermLoanDetails"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/SeniorSecuredTermLoanScheduleofTermLoanDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-74a4-8ac7-2090e0138149" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_019ff692-1faa-7e1d-bcdd-43cb61cb2e53" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-74a4-8ac7-2090e0138149" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_019ff692-1faa-7e1d-bcdd-43cb61cb2e53" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019ff692-1faa-7709-a573-222c8a3364f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019ff692-1faa-7e1d-bcdd-43cb61cb2e53" xlink:to="loc_us-gaap_CreditFacilityAxis_019ff692-1faa-7709-a573-222c8a3364f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019ff692-1faa-7709-a573-222c8a3364f8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019ff692-1faa-7709-a573-222c8a3364f8" xlink:to="loc_us-gaap_CreditFacilityDomain_019ff692-1faa-7709-a573-222c8a3364f8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019ff692-1faa-7a9e-a49f-81c6d0976b5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019ff692-1faa-7709-a573-222c8a3364f8" xlink:to="loc_us-gaap_CreditFacilityDomain_019ff692-1faa-7a9e-a49f-81c6d0976b5f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019ff692-1faa-76dd-ba91-f5ee5652a6cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019ff692-1faa-7a9e-a49f-81c6d0976b5f" xlink:to="loc_us-gaap_SecuredDebtMember_019ff692-1faa-76dd-ba91-f5ee5652a6cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019ff692-1faa-73b4-8f06-c6604b20220c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019ff692-1faa-7e1d-bcdd-43cb61cb2e53" xlink:to="loc_us-gaap_DebtInstrumentAxis_019ff692-1faa-73b4-8f06-c6604b20220c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ff692-1faa-73b4-8f06-c6604b20220c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ff692-1faa-73b4-8f06-c6604b20220c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ff692-1faa-73b4-8f06-c6604b20220c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ff692-1faa-7cd9-99bb-818e8f0297af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ff692-1faa-73b4-8f06-c6604b20220c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ff692-1faa-7cd9-99bb-818e8f0297af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_TheLoanAgreementMember_019ff692-1faa-7c89-a47e-6e15a5b6e9a2" xlink:href="mlys-20260930.xsd#mlys_TheLoanAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ff692-1faa-7cd9-99bb-818e8f0297af" xlink:to="loc_mlys_TheLoanAgreementMember_019ff692-1faa-7c89-a47e-6e15a5b6e9a2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019ff692-1faa-7c35-bd78-4f39e3e5c6b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019ff692-1faa-7e1d-bcdd-43cb61cb2e53" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019ff692-1faa-7c35-bd78-4f39e3e5c6b3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ff692-1faa-7c35-bd78-4f39e3e5c6b3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ff692-1faa-7c35-bd78-4f39e3e5c6b3" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ff692-1faa-7c35-bd78-4f39e3e5c6b3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ff692-1faa-707c-bc7c-d0dca4e59792" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ff692-1faa-7c35-bd78-4f39e3e5c6b3" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ff692-1faa-707c-bc7c-d0dca4e59792" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019ff692-1faa-7602-9d31-9f23f307600d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ff692-1faa-707c-bc7c-d0dca4e59792" xlink:to="loc_us-gaap_LineOfCreditMember_019ff692-1faa-7602-9d31-9f23f307600d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019ff692-1faa-7cbf-9c82-8569d5ac8048" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-74a4-8ac7-2090e0138149" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019ff692-1faa-7cbf-9c82-8569d5ac8048" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_019ff692-1faa-7d07-9aed-953634af2508" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-74a4-8ac7-2090e0138149" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_019ff692-1faa-7d07-9aed-953634af2508" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019ff692-1faa-7d27-ae89-1caa5349b64e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-74a4-8ac7-2090e0138149" xlink:to="loc_us-gaap_LongTermDebt_019ff692-1faa-7d27-ae89-1caa5349b64e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_019ff692-1faa-77d9-b9ef-0b36c18eff0b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-74a4-8ac7-2090e0138149" xlink:to="loc_us-gaap_LongTermDebtCurrent_019ff692-1faa-77d9-b9ef-0b36c18eff0b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_019ff692-1faa-7ef6-b805-57729cf95686" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-74a4-8ac7-2090e0138149" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_019ff692-1faa-7ef6-b805-57729cf95686" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_019ff692-1faa-7545-9824-2a4b4e7d12e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-74a4-8ac7-2090e0138149" xlink:to="loc_us-gaap_DebtInstrumentTerm_019ff692-1faa-7545-9824-2a4b4e7d12e4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_DebtInstrumentBasisFloor_019ff692-1faa-788d-82f0-04c6490aa589" xlink:href="mlys-20260930.xsd#mlys_DebtInstrumentBasisFloor"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-74a4-8ac7-2090e0138149" xlink:to="loc_mlys_DebtInstrumentBasisFloor_019ff692-1faa-788d-82f0-04c6490aa589" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ff692-1faa-714c-8d5b-347dd04da165" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019ff692-1faa-74a4-8ac7-2090e0138149" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ff692-1faa-714c-8d5b-347dd04da165" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/CapitalStockScheduleofAuthorizedSharesofCommonStockDetails" xlink:type="simple" xlink:href="mlys-20260930.xsd#CapitalStockScheduleofAuthorizedSharesofCommonStockDetails"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/CapitalStockScheduleofAuthorizedSharesofCommonStockDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019ff692-1faa-764c-b9fe-1e0f8d5506ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019ff692-1faa-772f-bb6e-9c0b49132314" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019ff692-1faa-764c-b9fe-1e0f8d5506ac" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019ff692-1faa-772f-bb6e-9c0b49132314" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019ff692-1faa-7fec-a882-c4eec01e68e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019ff692-1faa-772f-bb6e-9c0b49132314" xlink:to="loc_us-gaap_AwardTypeAxis_019ff692-1faa-7fec-a882-c4eec01e68e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-7fec-a882-c4eec01e68e5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019ff692-1faa-7fec-a882-c4eec01e68e5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-7fec-a882-c4eec01e68e5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-719b-9a35-50a4f92df448" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019ff692-1faa-7fec-a882-c4eec01e68e5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-719b-9a35-50a4f92df448" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019ff692-1faa-7d0c-8941-b277bb1d723a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-719b-9a35-50a4f92df448" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019ff692-1faa-7d0c-8941-b277bb1d723a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019ff692-1faa-701e-b884-bf47d4cec87a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-719b-9a35-50a4f92df448" xlink:to="loc_us-gaap_EmployeeStockMember_019ff692-1faa-701e-b884-bf47d4cec87a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019ff692-1faa-7b14-8941-787852ee636b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-719b-9a35-50a4f92df448" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019ff692-1faa-7b14-8941-787852ee636b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019ff692-1faa-7c20-bd9c-8f8390d95b11" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019ff692-1faa-772f-bb6e-9c0b49132314" xlink:to="loc_us-gaap_PlanNameAxis_019ff692-1faa-7c20-bd9c-8f8390d95b11" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019ff692-1faa-7c20-bd9c-8f8390d95b11_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019ff692-1faa-7c20-bd9c-8f8390d95b11" xlink:to="loc_us-gaap_PlanNameDomain_019ff692-1faa-7c20-bd9c-8f8390d95b11_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019ff692-1faa-7a44-9939-4b1a8ec26d81" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019ff692-1faa-7c20-bd9c-8f8390d95b11" xlink:to="loc_us-gaap_PlanNameDomain_019ff692-1faa-7a44-9939-4b1a8ec26d81" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_EquityIncentivePlan2023Member_019ff692-1faa-72f9-904b-a090aecf9451" xlink:href="mlys-20260930.xsd#mlys_EquityIncentivePlan2023Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019ff692-1faa-7a44-9939-4b1a8ec26d81" xlink:to="loc_mlys_EquityIncentivePlan2023Member_019ff692-1faa-72f9-904b-a090aecf9451" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_EquityIncentivePlan2025Member_019ff692-1faa-73ad-834b-1af8003683b6" xlink:href="mlys-20260930.xsd#mlys_EquityIncentivePlan2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019ff692-1faa-7a44-9939-4b1a8ec26d81" xlink:to="loc_mlys_EquityIncentivePlan2025Member_019ff692-1faa-73ad-834b-1af8003683b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_019ff692-1faa-7ef5-9cef-2bd0ea6e5b93" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019ff692-1faa-772f-bb6e-9c0b49132314" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_019ff692-1faa-7ef5-9cef-2bd0ea6e5b93" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_019ff692-1faa-7ef5-9cef-2bd0ea6e5b93_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_019ff692-1faa-7ef5-9cef-2bd0ea6e5b93" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_019ff692-1faa-7ef5-9cef-2bd0ea6e5b93_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_019ff692-1faa-70d3-975a-4caeda6a721b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_019ff692-1faa-7ef5-9cef-2bd0ea6e5b93" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_019ff692-1faa-70d3-975a-4caeda6a721b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_PreFundedWarrantMember_019ff692-1faa-7f27-8d25-1e5df15a05d5" xlink:href="mlys-20260930.xsd#mlys_PreFundedWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_019ff692-1faa-70d3-975a-4caeda6a721b" xlink:to="loc_mlys_PreFundedWarrantMember_019ff692-1faa-7f27-8d25-1e5df15a05d5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019ff692-1faa-7b61-b7ea-c1ee887f72ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019ff692-1faa-764c-b9fe-1e0f8d5506ac" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019ff692-1faa-7b61-b7ea-c1ee887f72ad" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/CapitalStockNarrativeDetails" xlink:type="simple" xlink:href="mlys-20260930.xsd#CapitalStockNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/CapitalStockNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019ff692-1faa-72e8-9430-aaee4b52315b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019ff692-1faa-75f2-b7c6-5b3be6303f6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019ff692-1faa-72e8-9430-aaee4b52315b" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019ff692-1faa-75f2-b7c6-5b3be6303f6d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1faa-72bc-a3f7-ecd7a0a5f570" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019ff692-1faa-75f2-b7c6-5b3be6303f6d" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1faa-72bc-a3f7-ecd7a0a5f570" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1faa-72bc-a3f7-ecd7a0a5f570_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1faa-72bc-a3f7-ecd7a0a5f570" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1faa-72bc-a3f7-ecd7a0a5f570_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1faa-7f00-a736-3d06fac71772" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019ff692-1faa-72bc-a3f7-ecd7a0a5f570" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1faa-7f00-a736-3d06fac71772" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_PublicStockOfferingMember_019ff692-1faa-7623-9e67-b45aee726f40" xlink:href="mlys-20260930.xsd#mlys_PublicStockOfferingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1faa-7f00-a736-3d06fac71772" xlink:to="loc_mlys_PublicStockOfferingMember_019ff692-1faa-7623-9e67-b45aee726f40" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_AtTheMarketAgreementMember_019ff692-1faa-7740-89a3-a8afebeeff71" xlink:href="mlys-20260930.xsd#mlys_AtTheMarketAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1faa-7f00-a736-3d06fac71772" xlink:to="loc_mlys_AtTheMarketAgreementMember_019ff692-1faa-7740-89a3-a8afebeeff71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_PriorATMAgreementMember_019ff692-1faa-7d0f-b142-0c3153a3cc35" xlink:href="mlys-20260930.xsd#mlys_PriorATMAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019ff692-1faa-7f00-a736-3d06fac71772" xlink:to="loc_mlys_PriorATMAgreementMember_019ff692-1faa-7d0f-b142-0c3153a3cc35" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_01a1150d-256c-7f8a-90a6-9798a2fea965" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019ff692-1faa-75f2-b7c6-5b3be6303f6d" xlink:to="loc_srt_CounterpartyNameAxis_01a1150d-256c-7f8a-90a6-9798a2fea965" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a1150d-256c-7f8a-90a6-9798a2fea965_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_01a1150d-256c-7f8a-90a6-9798a2fea965" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a1150d-256c-7f8a-90a6-9798a2fea965_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a1150d-256c-7010-8dff-53d955e47d1b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_01a1150d-256c-7f8a-90a6-9798a2fea965" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a1150d-256c-7010-8dff-53d955e47d1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_TanabeMember_01a1150d-256c-7115-8770-c7609c4e84b5" xlink:href="mlys-20260930.xsd#mlys_TanabeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a1150d-256c-7010-8dff-53d955e47d1b" xlink:to="loc_mlys_TanabeMember_01a1150d-256c-7115-8770-c7609c4e84b5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfArrangementAxis_01a1150d-256c-790d-8aaa-93d9df837603" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019ff692-1faa-75f2-b7c6-5b3be6303f6d" xlink:to="loc_us-gaap_TypeOfArrangementAxis_01a1150d-256c-790d-8aaa-93d9df837603" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_01a1150d-256c-790d-8aaa-93d9df837603_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_01a1150d-256c-790d-8aaa-93d9df837603" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_01a1150d-256c-790d-8aaa-93d9df837603_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_01a1150d-256c-7004-9377-14bb56e7bdc7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_01a1150d-256c-790d-8aaa-93d9df837603" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_01a1150d-256c-7004-9377-14bb56e7bdc7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember_01a1150d-256c-78a2-bd16-6bebac9885bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_01a1150d-256c-7004-9377-14bb56e7bdc7" xlink:to="loc_us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember_01a1150d-256c-78a2-bd16-6bebac9885bc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019ff692-1faa-78d8-9ad5-a2a08ef89970" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019ff692-1faa-75f2-b7c6-5b3be6303f6d" xlink:to="loc_srt_RangeAxis_019ff692-1faa-78d8-9ad5-a2a08ef89970" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ff692-1faa-78d8-9ad5-a2a08ef89970_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019ff692-1faa-78d8-9ad5-a2a08ef89970" xlink:to="loc_srt_RangeMember_019ff692-1faa-78d8-9ad5-a2a08ef89970_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ff692-1faa-7b2a-b15a-eefbcdd98c44" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019ff692-1faa-78d8-9ad5-a2a08ef89970" xlink:to="loc_srt_RangeMember_019ff692-1faa-7b2a-b15a-eefbcdd98c44" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019ff692-1faa-7095-ad28-ee554c841ffa" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ff692-1faa-7b2a-b15a-eefbcdd98c44" xlink:to="loc_srt_MaximumMember_019ff692-1faa-7095-ad28-ee554c841ffa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019ff692-1faa-7c68-9ab8-d9576a80ac69" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019ff692-1faa-72e8-9430-aaee4b52315b" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019ff692-1faa-7c68-9ab8-d9576a80ac69" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019ff692-1faa-7758-a204-981fcf8131c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019ff692-1faa-72e8-9430-aaee4b52315b" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019ff692-1faa-7758-a204-981fcf8131c9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019ff692-1faa-75ce-a271-3ad99b551c02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019ff692-1faa-72e8-9430-aaee4b52315b" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019ff692-1faa-75ce-a271-3ad99b551c02" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_CollaborativeArrangementRightsAndObligationsCommercialSaleMaximumAggregateMilestoneOneTimePayments_019ff692-1faa-7673-9122-8f69dfcd22d2" xlink:href="mlys-20260930.xsd#mlys_CollaborativeArrangementRightsAndObligationsCommercialSaleMaximumAggregateMilestoneOneTimePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019ff692-1faa-72e8-9430-aaee4b52315b" xlink:to="loc_mlys_CollaborativeArrangementRightsAndObligationsCommercialSaleMaximumAggregateMilestoneOneTimePayments_019ff692-1faa-7673-9122-8f69dfcd22d2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_SaleOfStockConsiderationReceivableOnTransaction_019ff692-1faa-788c-a691-d228e78a0aa6" xlink:href="mlys-20260930.xsd#mlys_SaleOfStockConsiderationReceivableOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019ff692-1faa-72e8-9430-aaee4b52315b" xlink:to="loc_mlys_SaleOfStockConsiderationReceivableOnTransaction_019ff692-1faa-788c-a691-d228e78a0aa6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_SaleOfStockPricePerShareWeightedAveragePrice_019ff692-1faa-74a6-beec-a36c8d326dfb" xlink:href="mlys-20260930.xsd#mlys_SaleOfStockPricePerShareWeightedAveragePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019ff692-1faa-72e8-9430-aaee4b52315b" xlink:to="loc_mlys_SaleOfStockPricePerShareWeightedAveragePrice_019ff692-1faa-74a6-beec-a36c8d326dfb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_MaximumAggregateOfferingPriceOfRemainingCommonSharesAvailableForSale_019ff692-1faa-7dcf-aef9-542125afc7bc" xlink:href="mlys-20260930.xsd#mlys_MaximumAggregateOfferingPriceOfRemainingCommonSharesAvailableForSale"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019ff692-1faa-72e8-9430-aaee4b52315b" xlink:to="loc_mlys_MaximumAggregateOfferingPriceOfRemainingCommonSharesAvailableForSale_019ff692-1faa-7dcf-aef9-542125afc7bc" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019ff692-1faa-7fb6-8132-67657acda015" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019ff692-1faa-72e8-9430-aaee4b52315b" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019ff692-1faa-7fb6-8132-67657acda015" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/StockBasedCompensationScheduleofOptionsbyPlanDetails" xlink:type="simple" xlink:href="mlys-20260930.xsd#StockBasedCompensationScheduleofOptionsbyPlanDetails"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/StockBasedCompensationScheduleofOptionsbyPlanDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-1faa-7b82-bc26-71af7dac2092" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-1faa-7957-a520-b17141a793b2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-1faa-7b82-bc26-71af7dac2092" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-1faa-7957-a520-b17141a793b2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019ff692-1faa-79ed-9a2c-b3b3a6b2c4a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-1faa-7957-a520-b17141a793b2" xlink:to="loc_us-gaap_PlanNameAxis_019ff692-1faa-79ed-9a2c-b3b3a6b2c4a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019ff692-1faa-79ed-9a2c-b3b3a6b2c4a2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019ff692-1faa-79ed-9a2c-b3b3a6b2c4a2" xlink:to="loc_us-gaap_PlanNameDomain_019ff692-1faa-79ed-9a2c-b3b3a6b2c4a2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019ff692-1faa-73cd-a3c1-a0671c2683eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019ff692-1faa-79ed-9a2c-b3b3a6b2c4a2" xlink:to="loc_us-gaap_PlanNameDomain_019ff692-1faa-73cd-a3c1-a0671c2683eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_A2023IncentiveAwardPlanMember_019ff692-1faa-70d2-8ea2-20b36bf4ba13" xlink:href="mlys-20260930.xsd#mlys_A2023IncentiveAwardPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019ff692-1faa-73cd-a3c1-a0671c2683eb" xlink:to="loc_mlys_A2023IncentiveAwardPlanMember_019ff692-1faa-70d2-8ea2-20b36bf4ba13" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_A2025IncentiveAwardPlanMember_019ff692-1faa-7544-ac4d-c1115303fbe8" xlink:href="mlys-20260930.xsd#mlys_A2025IncentiveAwardPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019ff692-1faa-73cd-a3c1-a0671c2683eb" xlink:to="loc_mlys_A2025IncentiveAwardPlanMember_019ff692-1faa-7544-ac4d-c1115303fbe8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_A2020EquityIncentivePlanMember_019ff692-1faa-7fe1-94da-c4f662185b77" xlink:href="mlys-20260930.xsd#mlys_A2020EquityIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019ff692-1faa-73cd-a3c1-a0671c2683eb" xlink:to="loc_mlys_A2020EquityIncentivePlanMember_019ff692-1faa-7fe1-94da-c4f662185b77" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019ff692-1faa-762b-a14f-a9bbfeafbf73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-1faa-7957-a520-b17141a793b2" xlink:to="loc_us-gaap_AwardTypeAxis_019ff692-1faa-762b-a14f-a9bbfeafbf73" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-762b-a14f-a9bbfeafbf73_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019ff692-1faa-762b-a14f-a9bbfeafbf73" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-762b-a14f-a9bbfeafbf73_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-71ea-bf29-e67a02f5be28" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019ff692-1faa-762b-a14f-a9bbfeafbf73" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-71ea-bf29-e67a02f5be28" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019ff692-1faa-779e-a5b5-fb32953ba40a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-71ea-bf29-e67a02f5be28" xlink:to="loc_us-gaap_RestrictedStockMember_019ff692-1faa-779e-a5b5-fb32953ba40a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019ff692-1faa-7f80-b724-85454ab908a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-71ea-bf29-e67a02f5be28" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019ff692-1faa-7f80-b724-85454ab908a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019ff692-1faa-7f7e-941f-fec09655e15b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-1faa-7b82-bc26-71af7dac2092" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019ff692-1faa-7f7e-941f-fec09655e15b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_019ff692-1faa-75ad-b7c6-8e60ef91de45" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-1faa-7b82-bc26-71af7dac2092" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_019ff692-1faa-75ad-b7c6-8e60ef91de45" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019ff692-1faa-74a4-b028-9963cf0e0083" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-1faa-7b82-bc26-71af7dac2092" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019ff692-1faa-74a4-b028-9963cf0e0083" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/StockBasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="mlys-20260930.xsd#StockBasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/StockBasedCompensationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-1faa-7c12-a9f7-0371ac9fa4db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-1faa-75ba-8acb-3c048727b7c1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-1faa-7c12-a9f7-0371ac9fa4db" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-1faa-75ba-8acb-3c048727b7c1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019ff692-1faa-7582-ad97-e998eabae6d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-1faa-75ba-8acb-3c048727b7c1" xlink:to="loc_us-gaap_AwardTypeAxis_019ff692-1faa-7582-ad97-e998eabae6d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-7582-ad97-e998eabae6d6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019ff692-1faa-7582-ad97-e998eabae6d6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-7582-ad97-e998eabae6d6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-79be-a7d6-f1a50bae5d4c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019ff692-1faa-7582-ad97-e998eabae6d6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-79be-a7d6-f1a50bae5d4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019ff692-1faa-7633-a682-904cbf9df4c0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019ff692-1faa-79be-a7d6-f1a50bae5d4c" xlink:to="loc_us-gaap_EmployeeStockMember_019ff692-1faa-7633-a682-904cbf9df4c0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019ff692-1faa-75d5-9808-ad90fe2752f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-1faa-75ba-8acb-3c048727b7c1" xlink:to="loc_us-gaap_PlanNameAxis_019ff692-1faa-75d5-9808-ad90fe2752f2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019ff692-1faa-75d5-9808-ad90fe2752f2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019ff692-1faa-75d5-9808-ad90fe2752f2" xlink:to="loc_us-gaap_PlanNameDomain_019ff692-1faa-75d5-9808-ad90fe2752f2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019ff692-1faa-7806-9693-d0246e8e4e08" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019ff692-1faa-75d5-9808-ad90fe2752f2" xlink:to="loc_us-gaap_PlanNameDomain_019ff692-1faa-7806-9693-d0246e8e4e08" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_A2023EmployeeStockPurchasePlanMember_019ff692-1faa-7818-ac2c-f2e30655dd22" xlink:href="mlys-20260930.xsd#mlys_A2023EmployeeStockPurchasePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019ff692-1faa-7806-9693-d0246e8e4e08" xlink:to="loc_mlys_A2023EmployeeStockPurchasePlanMember_019ff692-1faa-7818-ac2c-f2e30655dd22" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019ff692-204a-71b5-bf45-35e0f6b0da9d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-1faa-7c12-a9f7-0371ac9fa4db" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019ff692-204a-71b5-bf45-35e0f6b0da9d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_019ff692-204a-76c5-8a7e-085e5f22905a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-1faa-7c12-a9f7-0371ac9fa4db" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_019ff692-204a-76c5-8a7e-085e5f22905a" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/StockBasedCompensationScheduleofStockBasedCompensationReportedinStatementofOperationsDetails" xlink:type="simple" xlink:href="mlys-20260930.xsd#StockBasedCompensationScheduleofStockBasedCompensationReportedinStatementofOperationsDetails"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/StockBasedCompensationScheduleofStockBasedCompensationReportedinStatementofOperationsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-204a-7e53-8f34-044888bc2167" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-204a-79c8-9fcf-6d23b1c14697" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-204a-7e53-8f34-044888bc2167" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-204a-79c8-9fcf-6d23b1c14697" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019ff692-204a-7bb1-9893-92adfebc7278" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-204a-79c8-9fcf-6d23b1c14697" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019ff692-204a-7bb1-9893-92adfebc7278" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019ff692-204a-7de7-a7e4-b384d37284fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-204a-7e53-8f34-044888bc2167" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019ff692-204a-7de7-a7e4-b384d37284fd" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mineralystx.com/role/StockBasedCompensationScheduleofStockBasedCompensationReportedinStatementofOperationsDetails_1" xlink:type="simple" xlink:href="mlys-20260930.xsd#StockBasedCompensationScheduleofStockBasedCompensationReportedinStatementofOperationsDetails_1"/>
  <link:definitionLink xlink:role="http://www.mineralystx.com/role/StockBasedCompensationScheduleofStockBasedCompensationReportedinStatementofOperationsDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-204a-7e53-8f34-044888bc2167" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-204a-79c8-9fcf-6d23b1c14697" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-204a-7e53-8f34-044888bc2167" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ff692-204a-79c8-9fcf-6d23b1c14697" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019ff692-204a-7de7-a7e4-b384d37284fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ff692-204a-7e53-8f34-044888bc2167" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019ff692-204a-7de7-a7e4-b384d37284fd" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/TradingArrangementsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#TradingArrangmentsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/TradingArrangementsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllTradingArrangementsMember" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#ecd_AllTradingArrangementsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_SrinivasAkkarajuSamsaraBioCapitalLPPlanMember_01a157aa-76cc-72dd-85ed-965bb803c585" xlink:href="mlys-20260930.xsd#mlys_SrinivasAkkarajuSamsaraBioCapitalLPPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllTradingArrangementsMember" xlink:to="loc_mlys_SrinivasAkkarajuSamsaraBioCapitalLPPlanMember_01a157aa-76cc-72dd-85ed-965bb803c585" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_SrinivasAkkarajuSamsaraOpportunityFundLPPlanMember_01a157aa-e3e4-75ef-b14c-4d097d56dd2c" xlink:href="mlys-20260930.xsd#mlys_SrinivasAkkarajuSamsaraOpportunityFundLPPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllTradingArrangementsMember" xlink:to="loc_mlys_SrinivasAkkarajuSamsaraOpportunityFundLPPlanMember_01a157aa-e3e4-75ef-b14c-4d097d56dd2c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_AdamLevyTradingArrangement1Member_01a157a4-c1d0-7a95-9fb8-5256b103e299" xlink:href="mlys-20260930.xsd#mlys_AdamLevyTradingArrangement1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllTradingArrangementsMember" xlink:to="loc_mlys_AdamLevyTradingArrangement1Member_01a157a4-c1d0-7a95-9fb8-5256b103e299" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_AdamLevyTradingArrangement2Member_01a157a4-f050-7a5e-baae-f7f47f0ef630" xlink:href="mlys-20260930.xsd#mlys_AdamLevyTradingArrangement2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllTradingArrangementsMember" xlink:to="loc_mlys_AdamLevyTradingArrangement2Member_01a157a4-f050-7a5e-baae-f7f47f0ef630" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_SrinivasAkkarajuMember_01a157a2-abe2-7c99-bd21-0879d5eadd89" xlink:href="mlys-20260930.xsd#mlys_SrinivasAkkarajuMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_mlys_SrinivasAkkarajuMember_01a157a2-abe2-7c99-bd21-0879d5eadd89" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_JonCongletonMember_01a0f561-4d22-7d6a-b329-bbb191145837" xlink:href="mlys-20260930.xsd#mlys_JonCongletonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_mlys_JonCongletonMember_01a0f561-4d22-7d6a-b329-bbb191145837" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_AdamLevyMember_01a157a3-d467-7e2c-9b63-d83365bac22d" xlink:href="mlys-20260930.xsd#mlys_AdamLevyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_mlys_AdamLevyMember_01a157a3-d467-7e2c-9b63-d83365bac22d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mlys_EricWarrenMember_019ff692-1fa9-7195-bd7f-96e80fafab09" xlink:href="mlys-20260930.xsd#mlys_EricWarrenMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_mlys_EricWarrenMember_019ff692-1fa9-7195-bd7f-96e80fafab09" xlink:type="arc" order="5"/>
  </link:definitionLink>
</link:linkbase>
