The protein C pathway is a major anticoagulant mechanism that down-regulates the prothrombin- and intrinsic factor X (FX)-activating complexes via inactivation of their respective cofactors, activated factors V (FVa) and VIII (FVIIIa). Cofactor inactivation occurs by limited proteolysis at certain amino acid positions in FVa and in FVIIIa. These reactions are catalyzed by the serine protease activated protein C (APC) and stimulated by the APC cofactor protein S. Functional defects of the protein C pathway determine a plasma phenotype known as APC resistance, which is a prevalent and important risk factor for venous thrombosis. A plasma is termed 'APC resistant' when the addition of exogenous APC fails to prolong its clotting time in an activated partial thromboplastin time (aPTT) assay.