1 Introduction Adenomatous polyposis coli (APC) is a tumor suppressor gene that controls cell growth and division in the gastrointestinal tract. Germline pathogenic variants in the APC gene result in familial adenomatous polyposis (FAP), which can present with hundreds to thousands of colon adenomas or in a more attenuated form with fewer adenomas, sometimes referred to as attenuated FAP (AFAP), along with other features including gastric fundic gland polyps, gastric cancer, and duodenal cancer.