Both North American and European childhood ALL trials have shown that patients with lower TPMT activities and/or higher TGN levels have a lower relapse-risk (Lennard & Lilleyman, 1989; Schmeigelow et al, 1995; Balis et al, 1998; Relling et al, 1999b; Schmiegelow et al, 2009); TPMT heterozygotes have fewer relapses than those with a wild-type genotype (Schmiegelow et al, 2009; Lennard et al, 2015). The gene discussed is TPMT; the disease is acute lymphoblastic leukemia.