Coagulation factor XI is also known as F11, PTA and FXI. Factor XI triggers the middle phase of the intrinsic pathway of blood coagulation by activating factor IX. Factor XI is synthesized in the liver and circulates in the plasma as a disulfide bond-linked dimer complexed with high molecular weight kininogen. Selective cleavage of Arg-|-Ala and Arg-|-Val bonds in factor IX to form factor IXa. Factor XI is converted into XIa via either the contact phase of blood coagulation or thrombin-mediated activation on the platelet surface. The resulting XIa converts factor IX into IXa, which subsequently activates factor X into Xa.