Prothrombin is a protein produced by your liver. It is one of many factors in your blood that help it to clot appropriately. A coagulation (clotting) factor that is needed for the normal clotting of blood. A cascade of biochemical events leads to the formation of the final clot. In this cascade, prothrombin is a precursor to thrombin. Prothrombin is also known as thrombinogen and, in medical jargon, as pro. See also: Prothrombin mutation G20210A; and Prothrombin time. Prothrombin (factor II) is a vitamin K–dependent plasma protein synthesized in the liver. In the final common coagulation pathway, factor Xa within the prothrombinase complex (factor Va and phospholipid) cleaves an activation peptide (prothrombin fragment 1.2) from prothrombin to yield thrombin, which cleaves fibrinogen to produce fibrin, and activates platelets and factors V, VIII, XI, and XIII to form an insoluble hemostatic plug. Plasma prothrombin activity ranges from about 70% to 130%, and the amount of thrombin formed is directly proportional to the plasma prothrombin concentration.