Factor X (Mr 59,000) is a vitamin K-dependent plasma protein zymogen that plays a central role as the substrate for both the intrinsic (factor VIIa, tissue factor) and extrinsic (factor IXa, factor VIIIa) blood coagulation pathways. In the presence of cofactor factor Va, phospholipid, and Ca2+, activated factor X cleaves two peptide bonds in prothrombin to form thrombin.