Hereditary Diffuse Gastric Cancer is an inherited condition that sharply raises the lifetime risk of a specific type of stomach cancer called diffuse-type gastric cancer. Unlike sporadic (non-inherited) stomach cancer, which usually forms a visible lump or ulcer, diffuse-type cancer spreads as individual cancer cells through the stomach wall without forming a distinct tumour mass. This makes it harder to detect on a routine endoscopy, even when cancer is already present. Hereditary Diffuse Gastric Cancer is most often caused by a variation in the Cadherin 1 (CDH1) gene, which normally helps cells stick together properly; when it doesn’t work, cells can grow and spread abnormally. Some families with Hereditary Diffuse Gastric Cancer also carry mutations in the Catenin Alpha 1 gene.