Drug-metabolizing enzymes and membrane transporters work in concert to play crucial roles in drug absorption, distribution, and elimination. It is well recognized that genetic variation in drug-metabolizing enzymes contributes substantially to interindividual differences in drug response. With the notable exceptions of CYP1A1 and CYP2E1, genes encoding cytochrome P450s, which are involved in the metabolism of >80% of all drugs used in clinical practice, are highly polymorphic. Interethnic variation in the distribution and frequency of occurrence of variant alleles in drug-metabolizing enzymes is known to alter the rate of drug metabolism in vivo, resulting in interethnic variation in drug disposition and response.