Patients receiving PCI in the context of STEMI will undergo point-of-care genetic testing for the CYP2C19*2, *17, and ABCB1 3435 C>T alleles. CYP2C19*2 carriers and individuals homozygous for the ABCB1 3435 T allele will subsequently be randomized to prasugrel 10mg daily for 1 month or clopidogrel 150mg daily for 1 week followed by 75mg daily. The remaining individuals without an at-risk genotype will receive standard therapy with clopidogrel. At the end of the 1 month period, efficacy of the treatment strategies will be evaluated using VerifyNow platelet function testing. The effect of the CYP2C19*17 allele will be prospectively evaluated during the treatment period.