Protease-activated receptor (PAR)-4 is a member of a unique family of GPCRs. The protease-activated receptors (PARs) that are activated by proteolytic cleavage of the N-terminal domain of the receptor reveal a tethered ligand. The PAR family consists of 4 receptors; PAR1 and PAR3 are activated by thrombin, and PAR2 and PAR4 are activated by several serine proteases (Macfarlane et al., 2001). PAR4 is a recently identified low-affinity thrombin receptor that plays a pathophysiological role in many types of tissues including the lung. Mice lacking PAR4 are protected from mesenteric arteriole thrombosis, indicating that PAR4 is a potential target for treatment of thrombosis in humans.