Familial adenomatous polyposis (FAP) is an inherited disorder that is characterized by a greatly increased risk of cancer of the large intestine (colon) and rectum (collectively known as colorectal cancer). People with FAP have multiple precancerous (benign) growths (polyps) in the colon, and one or more of these polyps will likely develop into colorectal cancer. There are two forms of FAP: the classic type and the attenuated type.

Variants (also called mutations) in the APC gene cause FAP. The APC gene provides instructions for making a protein that acts as a tumor suppressor, which means that it keeps cells from growing and dividing too fast or in an uncontrolled way. In particular, the APC protein helps block a signaling pathway that promotes cell growth and division. The APC protein also interacts with parts of the cell to help ensure that the number of chromosomes in a cell is correct after the cell divides.

FAP has an incidence of 1 in 8,500 individuals. FAP accounts for about 0.5 percent of all cases of colorectal cancer.

FAP is inherited in an autosomal dominant pattern, which means one copy of the altered APC gene in each cell is sufficient to cause the disorder. Affected individuals have a 50 percent chance of passing on the APC gene variant to each child. In 75 to 80 percent of cases, an affected person has one parent with FAP.

Published Date: August 26, 2025
Published By: National Institutes of Health