There is pronounced individual variation in the human metabolism (eg, acetylation and hydroxylation) of drugs, in many cases due to genetic factors. The genetic basis of cytochrome P4502D6 gene defects has recently been elucidated, for instance. The cytochrome P4502D6 is absent from 7 percent of the Caucasian population, usually owing to the presence of either of two different mutations in the corresponding gene. PCR amplification of genomic DNA with allele-specific primers enables rapid prediction of the patient's drug metabolism capacity and the individualization of drug treatment with respect to dosage.