Lynch syndrome (LS), previously known as hereditary nonpolyposis colorectal cancer (HNPCC), is an autosomal dominant disorder caused by pathogenic germline variants in one of the DNA mismatch repair (MMR) genes, MLH1, MSH2, MSH6, or PMS2, or by deletions in the EPCAM gene (1–3). The gene discussed is MRC1; the disease is hereditary nonpolyposis colon cancer.