Cancer patients with low TPMT activity are at increased risk for thiopurine-induced myelotoxicity, in contrast to the potential need for higher-than-normal doses to achieve therapeutic levels of thiopurines in patients with high TPMT activity. The thiopurine drugs metabolized by TPMT have a relatively narrow therapeutic index, and are used to treat lifethreatening illnesses such as acute lymphoblastic leukemia or organ-transplant patients.