MLH1 and hereditary nonpolyposis colon cancer: Lynch syndrome, also known as hereditary non-polyposis colorectal cancer (HNPCC), is an autosomal dominant disorder characterized by germline mutations in mismatch repair (MMR) genes such as MLH1, MSH2, MSH6, and PMS2. Offspring of affected individuals have a 50% risk of inheriting the same pathogenic mutation.