MLH1 and hereditary nonpolyposis colon cancer: Lynch syndrome (also called hereditary non-polyposis colorectal cancer) is an autosomal dominant disorder that arises because of a germline mutation in at least one of four DNA mismatch repair (MMR) genes (MutL Homolog 1 (MLH1), MutS Homolog 2 (MSH2), MutS Homolog 6 (MSH6), and PMS1 Homolog 2 (PMS2)).