Protein S is a 69kD, secreted glycoprotein that belongs to a general group of vitamin K-dependent proteins. It is synthesized principally by hepatocytes, circulates both free (40%) and bound to C4bBP (60%), interacts with protein C to suppress coagulation, and binds TAM family RTKs during immune regulation. Mature human Protein S is 634 amino acids in length. It contains one Gla domain (aa42-87), four EGF-like repeats (aa117-283) and two laminin G-like domains (aa299-666). Over aa42-670, human Protein S shares 83%, 80%, and 79% aa sequence identity with porcine, mouse and canine Protein S, respectively.