Coagulation factor IX, also known as FIX and F9, one of the serine proteases, belongs to peptidase family S1. F9 is produced as a zymogen, an inactive precursor. It is processed to remove the signal peptide, glycosylated and then cleaved by factor XIa or factor VIIa to produce a two-chain form where the chains are linked by a disulfide bridge. It is inhibited by antithrombin. In addition, F9 expression increases with age in humans and mice.
Deficiency of this protein causes hemophilia B. Over 100 mutations of F9 have been described; some cause no symptoms, but many lead to a significant bleeding disorder. Recombinant F9 is used to treat hemophilia B, and is commercially available as BeneFIX and Alprolix. Some rare mutations of F9 result in elevated clotting activity, and can result in clotting diseases, such as deep vein thrombosis. Tranexamic acid may be of value in patients undergoing surgery who have inherited F9 deficiency in order to reduce the perioperative risk of bleeding.