The primary concerns of pharmacogenetics is the hereditary interindividual differences in response to drug therapy particularly variation in therapeutic efficacy and adverse drug reactions. However, soon after the field of pharmacogenetics emerged, this scope was broadened and the genetic polymorphisms were extensively studied not only in relation to known variation in drug response but also as susceptibility factors for diseases in general. In this aspects, both interindividual drug response and evaluation of genetic risk factors within the frame of gene-environment interactions are two important subjects in cancer pharmacogenetics.