Lynch Syndrome, also known as HNPCC (hereditary nonpolyposis colorectal cancer), is an autosomal dominant disease that is characterized by a mutation in the mismatch repair genes MSH or MLH. This defect in mismatch repair leads to DNA errors at microsatellite regions, which are regions of highly repetitive DNA. The increased error rate at microsatellite regions is termed microsatellite instability, and it is largely responsible for the many cancers seen in Lynch syndrome. Lynch syndrome is most notably characterized by colorectal cancer, which is why it is embodied by the HNPCC acronym: hereditary nonpolyposis "colorectal cancer." It typically occurs in the proximal colon, which helps differentiate it from a similar disease, FAP (familial adenomatous polyposis). Moreover, FAP is characterized by the presence of polyps, whereas HNPCC is not. Again, HNPCC stands for hereditary "nonpolyposis" colorectal cancer. Finally, Lynch patients also exhibit a number of other different cancer types. Notably, endometrial, ovarian, and skin cancers are commonly seen.