Prostasin (Prss8), also known as channel activating protease 1 (CAP1), is a trypsinlike serine peptidase, and plays important roles in epithelial physiology. It is originally purified as an active, soluble enzyme from human seminal fluid and is highly expressed in prostate, lung, kidney, salivary gland and pancreas. Prostasin is expressed as a glycosyl-phosphatidylinositol (GPI)-anchored membrane protein in prostate epithelial cells, and also exists as a secreted proteolytic enzyme possibly via tryptic cleavage of its COOH-terminal hydrophobic domain. Prostasin is found to activate the epithelial sodium channel (ENaC) which is tightly regulated and is critical for maintaining salt and fluid balance in the lung and kidney in both normal and pathological conditions. Accordingly, prostasin has been proposed as a target for therapeutic inhibition in cystic fibrosis. In addition, prostasin inhibits prostate and breast cancer cell invasion in vitro, suggesting a functional role as a suppressor of tumor invasion, as well as a regulator of gene expression during inflammation.