These findings do not support any change in the prescribing criteria (both genotypes start at the same standard protocol dose and titrate to toxicity) for the UK ALL trials with respect to the TPMT heterozygous patient; a similar conclusion to that reached with respect to mercaptopurine dosages in the German BFM protocols (Stanulla et al, 2005). Here, TPMT is linked to acute lymphoblastic leukemia.