Cellular receptors for blood proteases regulate chemotaxis, extracellular proteolysis, and growth behavior of normal and malignant cells. Effector cell protease receptor-1 (EPR1) is a receptor for the coagulation protease factor Xa. EPR1 is characterized by a cysteine-rich extracellular module, a single membrane-spanning domain, and a serine-rich cytoplasmic tail featuring at least 15 potential phosphorylation sites. EPR1 also contains 2 N-linked glycosylation sites, 4 O-linked glycosylation sites, and a chondroitin sulfate attachment site, which may provide anchoring for carbohydrate chains, EPR1 transfectants bind to factor Xa in a specific and saturable manner, and in the absence of factor V/Va promote prothrombin activation in a factor Xa concentration-dependent reaction. Activated platelets and megakaryocytes express EPR1. Both EPR1 and membrane-bound factor Va are thought to be required to mediate factor Xa binding to the activated platelet to form a functional prothrombinase complex.