APC-associated adenomatous polyposis (OMIM #175100) includes familial adenomatous polyposis (FAP), attenuated familial adenomatous polyposis (AFAP), and gastric carcinoma with proximal polyposis (gastric adenocarcinoma and proximal polyposis of the stomach, GAPPS). With FAP, starting in adolescence, it is most often the case for hundreds to thousands of intestinal polyps to occur – usually located in the colorectal region – that can progress if left untreated and lead to carcinomas. In addition, there is an increased risk that other malignant and benign tumors will develop.
AFAP is a variant of FAP with much fewer polyps and a later onset of carcinomas.
GAPPS is characterized by polyposis in the gastric fundus region and an increased risk of gastric carcinoma. The colon is only marginally involved.