FAP may be due to faults (mutations) in the APC gene. APC is a gene that helps to protect against bowel polyps and cancer. Everyone has two APC genes (one from their mother, and one from their father). If one of these genes is not working, this is known as having a faulty APC gene, or having an APC mutation.
People with FAP develop large numbers of polyps (usually more than 100) in the bowel. Polyps are not cancerous, but if left untreated bowel cancer will develop. Most people with FAP develop bowel polyps during their teenage years. Polyps or cancer may also occur in other parts of the digestive tract.
Some people have a milder form of FAP called Attenuated Familial Adenomatous Polyposis (AFAP). People with AFAP usually develop fewer polyps in the bowel but still have an increased chance of developing bowel cancer.