Inter-individual dosing of 6-mercaptopurine in the treatment of childhood acute lymphoblastic leukemia is thought to be dependent upon TPMT phenotype, although measurement of TPMT pre-treatment and TPMT genotyping have been shown to be not as useful as hoped, with concordance between genotype and phenotype being as low as 50% in some TPMT-heterozygous cohorts. The gene discussed is TPMT; the disease is acute lymphoblastic leukemia.