Factor V is an essential component in the blood coagulation cascade. Inherited or acquired deficiencies in factor V are rare causes of bleeding disorders. Factor V deficiency is also known as Owren disease. Dr. Paul Owren identified this defect in Norway in 1943. Using relatively primitive technology, he was able to deduce the existence of a fifth component required for fibrin formation, which he named factor V, thus beginning the era of Roman numerology for coagulation factors. Factor V is an essential component in the blood coagulation cascade. Factor V is synthesized in the liver and possibly in megakaryocytes. Factor V circulates in an inactive form. During coagulation, factor V is converted to the active cofactor, factor Va, via limited proteolysis by the serine protease a-thrombin. Factor V and activated factor Xa form the prothrombinase complex. The prothrombinase complex is responsible for the rapid conversion of the zymogen prothrombin to the active serine protease a-thrombin.