Lynch syndrome is a hereditary cancer syndrome caused by germline pathogenic variants in DNA mismatch repair genes (MMR) such as MLH1, MSH2, MSH6, PMS2, and EPCAM. Families with Lynch syndrome have a high lifetime risk of developing colorectal, endometrial, ovarian, small intestine, ureteral, and renal pelvis cancer, and tend to develop cancer at a young age. Here, MLH1 is linked to Lynch syndrome.