In agreement with previous UK ALL trials (Lennard et al, 1990, 2013, 2015) mercaptopurine metabolite accumulation differed by genotype with TPMT wild-type patients accumulating lower concentrations of the TGN active metabolites than TPMT heterozygotes (Table III) or TPMT-deficient patients. The gene discussed is TPMT; the disease is acute lymphoblastic leukemia.