Protein C is best known for its anticoagulant activity, the proteolytic inactivation of FVa and FVIIIa on negatively charged phospholipid membranes. This is enhanced by cofactors protein S and FV (Rosing et al. 1995, Norstrom et al. 2006). Inactivation of FVa involves APC-mediated cleavages at Arg306 and Arg506. The rapid cleavage at Arg506 is kinetically favored over cleavage at Arg306, but results only in partial inactivation of FVa, whereas the slower cleavage at Arg306 results in a complete loss of FVa function (Kalafatis et al. 1994, Nicolaes et al. 1995). Protein S accelerates factor Va inactivation by selectively promoting the slow cleavage at Arg306 (Rosing et al. 1995). A mutation of the APC cleavage sites in FV Arg506Gln a.k.a. FVLeiden is the most common identifiable hereditary risk factor for venous thrombosis among Caucasians (Camire 2011). APC also has a role in the inactivation of FVIIIa (Regan et al. 1994). Similar to FVa inactivation, FVIIIa is cleaved by APC at Arg336 in the A1 subunit and at Arg562 in the A2 subunit, with either resulting in a complete loss of cofactor activity (O'Brien et al. 2000, Manithody et al. 2003). Both protein S and FV but not FVa enhance inactivation of FVIIIa by APC (O'Brien et al. 2000,57). By acting on FVa and FVIIIa Protein C down-regulates both primary and secondary thrombin formation, delaying clot formation and diminishing activation of TAFI, enhanced susceptibility of the clot to fibrinolysis, respectively. The latter effects of APC on secondary thrombin formation is sometimes referred to as APC’s profibrinolytic effect (Bajzar et al. 1996).