HNPCC may technically present with polyps, so the "non-polyposis" part of HNPCC is really a relative lack of polyps -- HNPCC has much fewer polyps than does FAP.
Lynch Syndrome, also known as hereditary nonpolyposis colorectal cancer (HNPCC), 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 microsatellite instability (errors at regions of highly repetitive DNA), leading to the development of several cancers. These cancers classically include colorectal cancer involving the proximal colon, with a relative lack of polyps (distinguishing it from familial adenomatous polyposis, or FAP -- which has much more polyps). Other cancers observed include endometrial, ovarian, and skin cancers.
Find Lynch Syndrome and more DNA Repair Disorders among Pixorize's visual mnemonics for the USMLE Step 1 and NBME Shelf Exams.