F11 and Arterial thrombosis: Both epidemiological and animal studies indicate that the intrinsic coagulation factors, factor XI (FXI) and factor XII (FXII), contribute to the development of pathological thrombus formation.1, 2, 3, 4In mice, congenital deficiency of eitherF11orF12protected against experimentally induced arterial thrombosis.5, 6, 7Since these deficiencies are not associated with bleeding (FXII) or with a mild bleeding phenotype (FXI), respectively, the contribution of these intrinsic factors in normal hemostasis is limited.