Factor V (formerly referred to as accelerator globulin and labile factor) is a large glycoprotein (320 kDa) that is produced in the liver. The gene that encodes factor V (FV) is located on chromosome 1. A congenital deficiency of FV is a hemorrhagic disorder inherited as an autosomal recessive disease. The concentration of FV in plasma is typically 10 micrograms/milliliters and approximately 35 micrograms/milliliters in bovine plasma. FV is a pro-cofactor that is activated through limited proteolysis by thrombin, or by activated factor X in the presence of phospholipid surface. Other physiologic activators of FV include plasmin, neutrophil elastase and platelet calpain. The activated cofactor (FVa) is an essential component of the prothrombin activator complex, which consists of FVa, activated factor X, calcium and anionic phospholipid surface. The intact prothrombinase complex activates prothrombin to thrombin at a rate 300,000-fold greater than activated factor X alone. In a positive feedback loop, the thrombin generated accelerates its own generation by activating more FV to FVa. Thrombin also acts to down-regulate FVa indirectly by activating Protein C, which inactivates FVa cofactor activity.