Lynch syndrome (LS; MIM# 120435) is an autosomal dominant cancer predisposition, caused by a heterozygous inactivating germline defect in one of the major mismatch repair (MMR) genes: MLH1 (MIM# 120436), MSH2 (MIM# 609309), MSH6 (MIM# 600678), or PMS2 (MIM# 600259). The gene discussed is PMS2; the disease is cancer.