Familial adenomatous polyposis (FAP) is characterized by hundreds to thousands of adenomatous polyps in the colon which usually starts at adolescence. Familial adenomatous polyposis (FAP) is a rare, autosomal dominant condition caused by a defect in the adenomatous polyposis coli (APC) gene. FAP arises from germline mutations of the APC gene on chromosome 5q21.