Protein S is a vitamin K dependent protein. It is a physiological inhibitor of coagulation. It acts as a cofactor of activated protein C by promoting the inactivation of FVa and FVIIIa, prothrombin, of the prothrombinase complex, FX. A protein S deficiency can be either acquired (hepatocellular insufficiency, vitamin K deficiency, anti-protein S antibody, ...) or constitutional (heterozygous or homozygous deficiency) grouped into 2 types depending on whether the deficiency is quantitative (type I) or qualitative (type II).