P2RY1 binds ADP (Leon et al. 1996). It is one of two platelet ADP receptors, the other is P2RY12, that initiate platelet activation when stimulated in concert. P2RY1 signals through Gq, while P2Y12 signals through Gi. This results in mobilization of intracellular calcium ions via activation of phospholipase C, which leads to a change in platelet shape and probably to platelet aggregation (Schachter et al. 1997).