Although AFAP can often be clinically diagnosed based on the number of polyps in the colorectum (< 100) and a family history consistent with autosomal dominant inheritance, extracolonic manifestations (e.g., fundic gland polyposis, duodenal adenoma, osteoma, desmoid tumor, CHRPE), etc., identification of a pathogenic germline variant in the APC gene is useful for a definitive diagnosis. The gene discussed is APC; the disease is gastric adenocarcinoma and proximal polyposis of the stomach.