Pharmacogenetics deals with inherited differences in the response to drugs. The best-
recognized examples are genetic polymorphisms of drug-metabolizing enzymes, which
affect about 30% of all drugs. Loss of function of thiopurine S-methyltransferase (TPMT)
results in severe and life-threatening hematopoietic toxicity if patients receive standard
doses of mercaptopurine and azathioprine. Gene duplication of cytochrome P4502D6
(CYP2D6), which metabolizes many antidepressants, has been identified as a mechanism …