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Patent Pending Factor X, also known by the eponym Stuart–Prower factor or as prothrombinase, thrombokinase or thromboplastin, is an enzyme (EC 3.4.21.6) of the coagulation cascade. It is a serine endopeptidase (protease group S1, PA clan). Factor X is synthesized in the liver and requires vitamin K for its synthesis. Factor X is activated into factor Xa by both factor IX (with its cofactor, factor VIII in a complex known as intrinsic Xase) and factor VII with its cofactor, tissue factor (a complex known as extrinsic Xase). It is therefore the first member of the final common pathway or thrombin pathway.