MLH1 and Leigh syndrome: Colorectal cancer (CRC) is mostly a sporadic disease, but 5–10% of cases arise as a result of a genetic predisposition of which Lynch syndrome (LS) is the most prevalent.1 LS is caused by a heterozygous germline mutation in one of the DNA mismatch repair (MMR) genes, MSH2, MLH1, MSH6 and PMS2.