Factor Xa (EC 3.4.21.6) is a serine endopeptidase located at the confluence of the intrinsic and extrinsic pathways of the blood coagulation cascade, and composed of two disulfide-linked subunits that converts prothrombin to thrombin. Factor Xa cleaves after the arginine residue in its preferred cleavage site Ile-(Glu or Asp)-Gly-Arg and it will occasionally cleave at other basic residues. However, it will not cleave at a site followed by proline or arginine. fXa has emerged as an attractive target for developing safer anticoagulant drugs. Inhibition of fXa should prevent production of new thrombin without affecting its basal level, which should ensure primary hemostasis.