Hereditary gastric cancer refers to the occurrence of gastric cancer in a familial context and is described as two or more cases of gastric cancer in first or second degree relatives with at least one case diagnosed before the age of 50. Familial clustering is observed in 10% of all cases of gastric cancer, and includes hereditary diffuse gastric cancer (early onset diffuse-type gastric cancer), gastric adenocarcinoma and proximal polyposis of the stomach (see these terms) and familial intestinal gastric cancer (familial clustering of intestinal type gastric adenocarcinoma). Hereditary gastric cancer can also occur in other hereditary cancer syndromes such as Lynch syndrome, Li-Fraumeni syndrome, familial adenomatous polyposis and juvenile polyposis syndrome (see these terms).