Inhibition of the synthesis or activity of Factor X is the mechanism of action for many anticoagulants in use today. Warfarin, a synthetic derivative of coumarin, is the most widely used oral anticoagulant in the US. In some European countries, other coumarin derivatives (phenprocoumon and acenocoumarol) are used. These agents known as vitamin K antagonists (VKA), inhibit the vitamin K-dependent carboxylation of Factors II (prothrombin), VII, IX, X in the hepatocyte. This carboxylation after the translation is essential for the physiological activity.