Protease Activated Receptor (PAR) is a rhodopsin class receptor found in the cardiovascular system whose normal function is to participate in blood clotting through thrombin signaling. PARs are a subclass of GPCRs containing four members simply named PAR1, PAR2, PAR3, and PAR4. This receptor subclass is activated by a proteolytic cleavage of their extracellular domain by serine proteases such as thrombin and trypsin. When cleavage occurs, the tethered portion of the extracellular domain interacts with the transmembrane domain and activates cytoplasmic G proteins. Receptor activation of G proteins leads to a wide range of cellular responses such as calcium signaling, cell adhesion, cell migration, and gene transcription.