Pharmacogenetic studies in childhood ALL have associated toxicity to 6-MP to single nucleotide polymorphism (SNP) in genes coding for 6-MP metabolizing enzymes such as thiopurine S-methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPA) [5,6]. This evidence concerns the gene ITPA and acute lymphoblastic leukemia.