sentence: Related studies reported that the tolerance to 6-MP varies among each individual patient , in which the most common adverse effect of the drug is myelosuppression with the onset at 7 – 10 days , nadir at 14 days , and recovery occurring by 21 days.10 ,11 Pathogenesis of this discrepancy of the drug-related toxicity seems to be related to genetic polymorphisms in the genes involved in thiopurine metabolisms.12Genetic polymorphisms in the gene encoding thiopurine methyltransferase ( TPMT ) are widely accepted to be involved in thiopurine metabolisms resulting in 6-MP related myelotoxicity ; however , geographic variation of frequencies and distributions of variant TPMT alleles have been reported in which the allelic frequencies were much lower in Asian compared with Caucasian populations.13 – 15 On the other hand , genetic polymorphisms in other genes encoding enzymes involved in thiopurine metabolisms including inosine triphosphate pyrophosphohydrolase ( ITPA ) and nucleoside diphosphate linked moiety X-type motif 15 ( NUDT15 ) has been reported to be more evident in Asian compared with Caucasian populations.16 – 18 ITPase is an enzyme involved in the hydrolysis of thioinosine triphosphate ( TITP ) to thioinosine monophosphate ( TIMP ) in which the deficiency of this enzyme among patients receiving 6-MP could result in toxic accumulation of TITP.16 ,19 However , the clinical correlation between ITPA genotypes and 6-MP related toxicity remains controversial and is not routinely applied to clinical practice .