The F9 gene encodes coagulation factor IX, which circulates as an inactive zymogen until proteolytic release of its activation peptide allows it to assume the conformation of an active serine protease. Its role in the blood coagulation cascade is to activate factor X (F10) through interactions with calcium, membrane phospholipids, and factor VIII (F8). Factor IX and factor X both consist of 2 polypeptide chains referred to as the L (light) and H (heavy) chains. The H chain bears a structural resemblance to the polypeptide chain of the pancreatic serine protease trypsin (PRSS1). The L chain is covalently linked to the H chain by a single disulfide bond.