Many of the complexities of human drug response are sufficiently well understood to transform the field of pharmacogenetics from a descriptive science to a predictive science. Clinical application of these markers is currently limited by lack of knowledge about the effects of modifying genes, about their prevalence and risk contribution in different ethnogeographic populations, and by fragmentary information about how genetic factors interact with physiologic or pathologic and other environmental factors. Nevertheless, progress has been notable, as exemplified in the identification of genetic markers predictive of pharmacokinetic variation, and in markers predictive of outcome and therapeutic benefit in the treatment of cancer.