Approximately 10% of cases with gastric cancer (GC) exhibit familial clustering, however, only 1–3% of cases can be explained by two known hereditary syndromes: Hereditary Diffuse Gastric Cancer (HDGC) caused by CDH1 and CTNNA1 pathogenic germline variants; and Gastric Adenocarcinoma and Proximal Polyposis of the Stomach (GAPPS), caused by germline variants in APC 1B promoter. The gene discussed is CDH1; the disease is gastric cancer.