Monoclonal antibody PS7 is specific for human Protein S (PS), a vitamin K-dependent plasma glycoprotein with a molecular weight of approximately 70 kDa. In humans, PS is mainly synthesized by hepatocytes. Mature PS has a modular structure consisting of a Gla-domain containing a short aromatic stack, a region sensitive to cleavage by thrombin and factor Xa , 4 epidermal growth factor (EGF)-like domains, and a sex hormone-binding globulin-like region. The biologic function of PS thought to be of primary importance is its ability to enhance Activated Protein C-dependent proteolytic inactivation of factors Va and VIIIa, which are respectively the cofactors in the pro-thrombinase and tenase complexes of the coagulation cascade. In this way, PS plays an important role in regulating thrombin generation, and therefore controlling procoagulant activity. Crucial to the APC-dependent functions of PS is the high-affinity interaction between PS and negatively charged phospholipid surfaces, conferred by calcium-induced folding of the amino-terminal Gla-domain. Monoclonal antibody PS7 allows specific detection of cells carrying PS by flow cytometry.