Familial adenomatous polyposis is a rare but important disease because colorectal cancer develops before age 40 innearly all untreated patients. The trait is autosomal dominant. The APC gene ("FAP" had already been used in the geneticlexicon) was localized to chromosome 5q21 in 1991, and since then more than 100 different mutations have beenidentified. Genetic testing of at-risk individuals is currently available. Patients present with hundreds to thousands of polypof varying size and configuration in the colon and rectum. A subset of patients have attenuated FAP (previously known ashereditary flat adenoma syndrome) characterized by the presence of fewer polyps (usually < 100), usually less than 100,later onset of colon cancer, and characteristic different mutations of the APC gene. Syn: adenomatous polyposis coli.A long list of benign and malignant extracolonic manifestations are associated with familial adenomatous polyposis.rnedulloblastoma