Clopidogrel (also known as Plavix) is used commonly in patients to prevent heart attacks and conditions caused by blood clots. Although clopidogrel works in many individuals, some people do not respond as well to this drug. The variation in treatment response may be linked to genetics. This study will examine the effects of clopidogrel in a population in which sequencing for certain genes has been performed in order to determine the role that genes play in the response to various clopidogrel maintenance doses.