HNPCC, also known as Lynch syndrome I and II, is an autosomal dominant disease caused by mutations in 1 of at least 4 DNA mismatch repair genes. The tumor DNA of these patients has widespread instability in short repeat
sequences, termed “microsatellite instability.” They still develop cancer through the adenoma-carcinoma pathway.
Lynch syndrome I (hereditary site-specific colorectal cancer) is associated with an increased risk of multiple
colon carcinomas that occur earlier and tend to be located in the right colon. Lynch syndrome II ( cancer family
syndrome) has the same risk for colon cancer and is also associated with early onset of carcinomas occurring at
other sites (ovaries, endometrium, stomach, pancreas, bladder, larynx, brain).