Lynch syndrome (LS), previously known as hereditary nonpolyposis colorectal cancer (CRC), is an autosomal dominant disorder characterized by germline variants in the mismatch repair (MMR) gene (e.g., MLH1 and MSH2) with microsatellite instability (MSI), which leads to the development of CRC in 80% of cases with LS. The gene discussed is MSH2; the disease is Lynch syndrome.