TPMT and acute lymphoblastic leukemia: TPMT heterozygotes taking mercaptopurine were found to have a lower relapse risk in the Nordic Society for Paediatric Haematology and Oncology (NOPHO) ALL‐92 trial (Schmiegelow et al, 2009a) but the benefit was offset by a higher incidence of second cancers (Schmiegelow et al, 2009b).