High platelet reactivity on clopidogrel may be due to various reasons including polymorphism in the gene encoding the cytochrome P-450 2C19 enzyme (CYP2C19) which has been shown to contribute to the variability of platelet response to clopidogrel. CYP2C19 is a key enzyme in this activation process and our group was the first to describe an association between the presence of the loss of function CYP2C19 681G>A polymorphism (also called *2) with lower clopidogrel responsiveness in healthy subjects.