Dual antiplatelet therapy with aspirin and clopidogrel (Plavix®; Sanofi-Aventis U.S. LLC) is recommended both in patients after acute myocardial infarction (MI) and in patients undergoing percutaneous coronary intervention (PCI) with stenting. Clopidogrel is a prodrug that requires transformation to an active metabolite in the liver by the cytochrome P450 (CYP) enzymes. The CYP genes are highly polymorphic and recent data suggest that certain alleles, particularly of the CYP2C19 gene located on chromosome 10 at band q24.1 to q24.3, result in reduced metabolism of a variety of drugs, including clopidogrel. There are 4 distinct alleles that may contribute to poor drug metabolism phenotypes for CYP2C19, which are referred to as: CYP2C19*2, CYP2C19*3, CYP2C19*4, and CYP2C19*5. CYP2C19*2 and CYP2C19*3 are the most frequently occurring variants that appear to influence the effective conversion of clopidogrel to an active metabolite and the degree of platelet inhibition provided by a given dose of clopidogrel.