The development of this MS-stable and TMB-high tumor in this patient may be attributed to a germline mutation in MSH6. In Lynch syndrome (LS), germline mutations in MMR genes (MLH1, MSH2, MSH6, PMS2, and EPCAM) predispose individuals to various malignancies, including colorectal and endometrial cancers. The gene discussed is PMS2; the disease is Leigh syndrome.