P2Y1 is a receptor for ATP and ADP. P2Y1 contributes to platelet shape change. A number of P2Y1 receptor-specific antagonists, such as, MRS-2179, A3P5PS, A3P5P and A2P5P have been discovered. They inhibit calcium ion mobilization and shape change in platelets. P2Y1-deficient mice and mice treated with the P2Y1 antagonist MRS2179 displayed significantly less arterial thrombosis than their respective controls. Combination of P2Y1 deficiency with P2Y12 inhibition led to a significant additive effect.