The need to titrate treatment to adequate therapeutic levels is exacerbated by the cytotoxicity that can result from overdosing patients. In this comprehensive study, we evaluated the response of 946 individuals, with known thiopurine S-methyltransferase (TPMT) genotypes, to treatment with 6-MP and Aza. We determined the allelic frequencies of the most common TPMT alleles in a diverse cohort of individuals. The TPMT*1/TPMT*1 genotype was found to occur in 92.1% of the patient population, while the TPMT*1/TPMT*3A, TPMT*1/ TPMT*3C, and TPMT*1/TPMT*2 genotypes were found to occurin 6.0%, 1.8%, and 0.1% of the patient population, respectively. We evaluated how genotype affected therapeutic response and make safe dosing recommendation based on genotype read more..