Activation of protein C by thrombin-thrombomodulin. Thrombomodulin (TM) is present on all endothelial cells and serves as a cofactor to thrombin in the activation of protein C. The endothelium also contains the endothelial protein C receptor (EPCR) that binds the Gla domain of protein C and helps present protein C to the T/TM complex. The activated protein C (APC) then floats along with the bloodstream to control reactions of coagulation. Illustration by Marie Dauenheimer.