Clopidogrel is an inactive prodrug activated by several enzymes, including CYP2C19, to an active metabolite that inhibits the platelet adenosine diphosphate receptor, P2RY12 (29). CYP2C19 *2 (rs4244285) is the most common reduced-function (RF) variant; additional, rare variants mimic the *2 allele: *3 [rs4986893], *4 [rs28399504], *5 [rs56337013]) (30). An individual person's genotype can be characterized in 3 ways: 1) presence of at least 1 RF allele (carrier vs. noncarrier); 2) the number of RF alleles (i.e., 0, 1, 2); or 3) the predicted, total CYP2C19 enzymatic activity (Table 4). Carriers of *2—also referred to as intermediate metabolizers or poor metabolizers—produce lower active metabolite and have attenuated platelet inhibition (31). In general, there is a gene-dose effect where increasing number of RF alleles (i.e., 0, 1, 2, or extensive metabolizers, intermediate metabolizers, and poor metabolizers) predicts a decreasing amount of platelet inhibition (27,31- 32). Carriers of another variant *17 (rs3758581, ultrametabolizers) exhibit increased CYP2C19 activity, produce more active metabolite, and improved platelet inhibition, in most reports (33- 34).