Although no dose difference was identified from the medical records of the patients carrying either the heterozygous (*1/*3C) or wild type (*1/*1) genotype of TPMT, since myelotoxicity was significantly associated with patients heterozygous for the genetic polymorphism of TPMT gene, this study suggests the careful management of 6-MP therapy in ALL patients carrying this variant. Here, TPMT is linked to acute lymphoblastic leukemia.