PROS1, also known as protein S, is a vitamin K-dependent plasma protein that functions as a cofactor for the anticoagulant protease, activated protein C (APC) to inhibit blood coagulation. PROS1 has two isoforms: a free, functionally active form and an inactive form complexed with C4b-binding protein. Besides its anticoagulant function, PROS1 also acts as an agonist for the tyrosine kinase receptors Tyro3, Axl, and Mer. The endothelium expresses Tyro3, Axl, and Mer and produces protein S. The interaction of protein S with endothelial cells and particularly its effects on angiogenesis have not yet been analyzed.