P2Y12 is a G-protein-coupled receptor that responds to ADP. Importantly, ADP mediates platelet aggregation through its action on G-protein-coupled receptor subtypes. Platelets have a crucial role in the maintenance of normal haemostasis, and perturbations of this system can lead to pathological thrombus formation and vascular occlusion, resulting in stroke, myocardial infarction and unstable angina. Additionally, ADP released from damaged vessels and red blood cells induces platelet aggregation through activation of the integrin GPIIb/IIIa and subsequent binding of fibrinogen. ADP is also secreted from platelets on activation, providing positive feedback that potentiates the actions of many platelet activators. Not surprisingly in the ensuing decades, P2Y12 receptors have been identified as important targets for antithrombotic therapy[1].