Familial adenomatous polyposis: Abbreviated FAP. A syndrome characterized by the formation of thousands of polyps in the colon and rectum with colorectal cancer the inevitable consequence. Polyps can also occur in the stomach, duodenum and the terminal ileum.

The polyps most often begin to form at puberty. And colon cancer usually occurs 10 to 15 years thereafter. The average age of diagnosis of familial polyposis is 25 years of age, with cancers developing at age 20 to 30. However, cancers may arise anywhere from late childhood to the sixties.

The syndrome is an autosomal dominant disorder with high penetrance and highly variable expressivity. A person with the disease thus has a 50% chance of passing the gene on to each of their children. Most people who receive the gene almost always manifest the disease although the expression of the disease can vary markedly from person to person.

The gene that is mutated in this disorder, symbolized APC for adenomatous polyposis coli, has been characterized and its protein identified. A variety of mutations in the APC gene have also been identified. The APC gene maps to chromosome 5 in region 5q21-q22.

Gardner syndrome (with polyps of the colon, extra bowel tumors, especially osteomas, and a rather characteristic abnormality of the retina of the eye) is now known not to be a wholly different disease but rather to be a variant of FAP, caused by a mutation in the APC gene.

Familial adenomatous polyposis (FAP) has also been called multiple polyposis of the colon, hereditary polyposis coli, familial multiple polyposis, familial polyposis of the colon (FPC) and adenomatous polyposis coli. The designation familial adenomatous polyposis (FAP) is most often used today, based in part on the appreciation that the polyps are not confined to the colon.