Receptor for extracellular adenine nucleotides such as ADP. In platelets, binding to ADP leads to mobilization of intracellular calcium ions via activation of phospholipase C, a change in platelet shape, and ultimately platelet aggregation. Platelets from mutant mice are defective in ADP-mediated platelet aggregation, leading to prolonged bleeding time. Otherwise, mice have no visible phenotype and do not display spontaneous bleeding. Belongs to the G-protein coupled receptor 1 family.