Pharmacogenetics is the study of genetic variations on drug metabolizing enzymes and transporters. Pharmacogenetics is one of the first clinical applications of the Human Genome Project. Pharmacogenomics is the study of the role of interindividual genomics variability on drug response, efficacy, and metabolism. It correlates the effects of the entire expressed genome to the clinical usefulness and toxicity of a drug. Pharmacogenomics has the potential to change the way patients' therapy is optimized, allowing an era of "personalized medicine" in which patients will be divided into groups based on genetic markers that treatment outcomes.