For children with acute lymphoblastic leukaemia (ALL) treated with mercaptopurine, those individuals with lower TPMT activities and/or higher TGN levels have a lower relapse‐risk (Lennard & Lilleyman, 1989; Schmeigelow et al, 1995; Relling et al, 1999a). The gene discussed is TPMT; the disease is acute lymphoblastic leukemia.