MLH1 and Leigh syndrome: Lynch syndrome (LS) is the most common hereditary colorectal cancer sysndrome, caused by a heterozygous germline pathogenic variant (PV) within one of the DNA mismatch repair (MMR) genes MLH1 (MIM 609310), MSH2 (MIM 120435), MSH6 (MIM 614350), PMS2 (MIM 600259) [1] or deletion of the 3′ end of the EPCAM gene leading to epigenetic inactivation of neighboring MSH2 (MIM 185535) [2,3].