In a study by Relling and coworkers in 180 children with ALL receiving conventional doses of 6-mercaptopurin, the authors found that the cumulative incidence of toxicity was 100% for homozygous TPMT deficiency, 35% for heterozygous, and 7% for patients homozygous for allele *1 (Relling et al., 1999). Here, TPMT is linked to acute lymphoblastic leukemia.