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Experts debate the usefulness of genetic testing to help determine the optimal antiplatelet regimen after an acute coronary syndrome or stent placement. Patients who carry a loss-of-function CYP2C19 allele have reduced conversion of clopidogrel to its active metabolite and decreased inhibition of platelet activation, which prompted the FDA to issue a black-box warning that clopidogrel can be less effective in carriers than in noncarriers. Another protein, encoded by the ABCB1 gene, is important in thienopyridine absorption and might also influence the effects of these drugs. Two key questions drive the debate: (1) Does an individual's genetic profile affect his or her relative responses to clopidogrel, placebo, or other agents? (2) Does an …