Thiopurine S-methyltransferase (TPMT) is an enzyme involved in the normal metabolic inactivation of thiopurine drugs. Patients with intermediate or no TPMT activity are at risk of toxicity after receiving standard doses of thiopurine drugs and it was shown that inter-individual differences in response to these drugs is largely determined by genetic variation at the TPMT locThiopurine S-methyltransferase (TPMT) is an enzyme involved in the normal metabolic inactivation of thiopurine drugs. Patients with intermediate or no TPMT activity are at risk of toxicity after receiving standard doses of thiopurine drugs and it was shown that inter-individual differences in response to these drugs is largely determined by genetic variation at the TPMT locus