Presence of the CYP2C19*2 allele has been associated with a 1.5- to 6-fold increased risk of cardiovascular death, myocardial infarction, and stent thrombosis following PCI in patients treated with clopidogrel. These findings, recently bolstered by 2 separate meta-analyses, led the American Food and Drug Administration to issue a boxed warning for clopidogrel stating that poor metabolizers may not receive the full benefit of the drug. The ABCB1 3435 TT genotype has also been linked with increased adverse cardiovascular events in individuals treated with clopidogrel following PCI for an acute coronary syndrome. In contrast, the CYP2C19*17 gain-of-function allele appears to enhance the activity of clopidogrel and has been associated with reduced ischemic events but increased bleeding.