Lynch syndrome (LS) is a hereditary autosomal dominant condition, defined and caused by pathogenic germline variants in a DNA mismatch repair (MMR) gene: MLH1, MSH2, MSH6, PMS2 or deletion of EpCAM. Patients with LS have an increased lifetime risk of developing malignancies, e.g., the lifetime risk of colorectal cancer (CRC) and endometrial cancer ranges from approximately 60–80%, compared with 3–4% in the general population [1–3]. The gene discussed is MLH1; the disease is Leigh syndrome.