Pharmacogenetics is that discipline devoted to elucidating the genetic determinants of drug response. Particularly in the area of drug metabolism, many genes exhibit genetic polymorphism; that is, a stable percentage of the population (which generally differs by ethnic group) is deficient in the functional expression of the enzyme involved, and the deficiency is typically inherited as an autosomal recessive trait. With currently known polymorphisms in drug metabolism, the percentage of homozygous deficient individuals ranges from 0.3% to as many as 90% of the population, depending on the enzyme and the ethnic group.