Recombinant DNA technology is a major innovation in medicine and is increasingly applied to study the mechanisms of inherited variations in drug response at the gene level. Three of these techniques are of particular importance to pharmacogenetics and for the study of the diversity of human genes. (1) Restriction analysis of genomic DNA, (2) enzymatic amplification of DNA by the polymerase chain reaction, and (3) the expression of cDNAs in cell culture. With these techniques large populations can be screened, normal and mutant DNA can be obtained from extremely small tissue samples, and functional expression of cDNAs allows the rapid search for potential new substrates of a polymorphic enzyme.