Gastric cancer is a major cause of cancer-related mortality worldwide and is thought to be responsible for about 10% of cancer-related deaths across the globe. A small proportion of all gastric cancers arise because of a known hereditary syndrome, the most common of which is hereditary diffuse gastric cancer (HDGC). This is an autosomal dominant genetic disease characterized by an increased risk of developing diffuse gastric cancer at a young age. The gene responsible for HDGC is CDH1, also known as E-cadherin, a germline mutation conferring an 80% risk of developing gastric cancer during the lifetime of the carrier. Females with germline CDH1 mutations face an additional risk of developing lobular breast cancer, with a reported cumulative risk of 60% by the age of 80 years.