Coagulation Factor VIII, also known as FVIII and F8, is a member of the multicopper oxidase family. Coagulation Factor VIII is a cofactor for factor IXa which, in the presence of Ca+2 and phospholipids, converts factor X to the activated form Xa. It contains 3 F5/8 type A domains, 2 F5/8 type C domains and 6 plastocyanin-like domains. FVIII is synthesized in the liver, and perhaps in other tissues. It is a coagulation cofactor which circulates bound to von Willebrand factor and is part of the intrinsic coagulation pathway. It is a macromolecular complex composed of two separate entities, one of which, when deficient, results in hemophilia A, and the other, when deficient, results in von Willebrand's disease. Hemophilia A is a disorder of blood coagulation characterized by a permanent tendency to hemorrhage. About 50% of patients have severe hemophilia resulting in frequent spontaneous bleeding into joints, muscles and internal organs. Less severe forms are characterized by bleeding after trauma or surgery.