This is contrary to the results seen in paediatric ALL patients, where the greatest efficacy is seen in the heterozygote TPMT group and where dose escalation of 6MP has been shown to be of benefit.27 However, the main methylation product, 6MMP, has been shown to be a strong inhibitor of purine de novo synthesis and also causes immunosuppression,27 and therefore this group (high/high TPMT status) may have thus had greater anticancer effects from treatment. This evidence concerns the gene TPMT and acute lymphoblastic leukemia.