Calpain-9 (CAPN9) belongs to the calpain family, which consists of a group of calcium-dependent cysteine proteases that regulate functions of substrates by proteolysis [, ]. Calpain-9 and Calpain-8 are predominantly expressed in the gastrointestinal tract and are restricted to the gastric surface mucus (pit) cells in the stomach. They are involved in gastric mucosal defence []. Calpain 9 is a tumor suppressor that can be regarded as a potential prognosis indicator for clinical outcomes in gastric cancer []and may play a role in breast cancer [].
