Hereditary nonpolyposis colorectal cancer (HNPCC, is subdivided into (1) Lynch syndrome I, or site-specific colonic cancer, and (2) Lynch syndrome II, or extracolonic cancer, particularly carcinoma of the stomach, endometrium (see 608089), biliary and pancreatic system, and urinary tract (Lynch and Lynch, 1979Lynch et al., 1985Mecklin and Jarvinen, 1991). HNPCC disorders show a proclivity to early onset, predominant proximal location of colon cancer, a dominant pattern of inheritance, an excess of multiple primary cancers, and significantly improved survival when compared stage for stage with the American College of Surgeons Audit Series.

Lynch et al. (1991) estimated that hereditary nonpolyposis colorectal cancer accounts for about 4 to 6% of colorectal cancer. The minimum criterion of HNPCC is that colorectal carcinoma is diagnosed and histologically verified in at least 3 relatives belonging to 2 or more successive generations. Moreover, the age of onset should be less than 50 years in at least 1 patient.

The Muir-Torre syndrome (MRTES; 158320) is a form of Lynch syndrome II associated with sebaceous skin tumors.