MLH1 and Leigh syndrome: Lynch syndrome (LS) is caused by germline mutations in one of the mismatch repair genes (MLH1, MSH2, MSH6, or PMS2) and is associated with increased cancer risk, including colorectal cancer (CRC), endometrial cancer (EC), and ovarian cancer (OC). Cancer surveillance by colonoscopy has been shown to reduce mortality from CRC. Gynecological surveillance by transvaginal ultrasonography, hysteroscopy, and endometrial biopsy is of unproven benefit. The age at initiation of cancer surveillance is also controversial. The debate is complicated by the rarity of LS and the lack of LS data.