The PROS1 gene encodes Protein S, a ~70 kDa vitamin-K dependent plasma glycoprotein mainly synthesized by human hepatocytes. Protein S is a non-enzymatic cofactor in the degradation of coagulation factors Va and VIIIa and thus plays an anticoagulant role. Approximately 60% of Protein S circulates in complex with C4b-binding protein. Free Protein S is a mixture of monomers and multimers. The 652 aa Protein S proprotein is heavily modified with 11 carboxyglutamates, one hydroxyaspartate, disulfides and glycosylation. Human Protein S shows 80% aa identity with mouse Protein S.