PAR-3 is a high-affinity thrombin receptor. PAR-3 mRNA is expressed in the cutaneous mast cells of humans. Protease-Activated Receptors (PARs) have been studied for their roles in itch and their itch-associated response through histamine-dependent/independent pathways have been reported. PAR-3 has been shown not to induce itching alone but possibly in conjunction with PAR-4. Co-expression of PAR-3 and PAR-4 enhances thrombin action suggesting that PAR-3 alone does not mediate transmembrane signaling but instead functions as a cofactor to activate PAR-4.