Protein C is a vitamin-K-dependent glycoprotein and plasma proenzyme of a serine protease that plays a key role in the down-regulation of blood coagulation. It is activated in vivo by the thrombin-thrombomodulin complex on the surface of intact endothelial cells. Activated protein C (APC) functions as a circulating anticoagulant through proteolytic cleavage and inactivation of the coagulation factors Va and VIIIa. The cleavage occurs at three sites in the heavy chain of each protein. The anticoagulant activity of APC is potentiated by the free form of Protein S (about 60% of PS in plasma is bound to C4bBP, and 40% is in free form) and FV. APC Resistance is actually due to a defect in the protein C pathway, in the factor V molecule as opposed to the activated protein C molecule. APC Resistance is an autosomal dominant hereditary defect mainly due to a point mutation resulting in an amino acid change in the FV gene (Ag506 to Gln mutation, or Factor V Leiden mutation). The mutation destroys one of the three cleavage sites, rendering FVa partially resistant to APC-mediated degradation. APC resistance occurs in 3-5% of the general population, but varies largely in different parts of the world. Up to 90% of APC resistance cases are due to the Factor V:Q506 gene mutation. The relative risk of DVT for carriers of the FV:Q506 mutation is estimated to be 8-fold for heterozygotes and 80-fold for homozygotes.