The TPMT*3B allele is assigned as a no function allele by CPIC. Patients with the TPMT*3B allele, in combination with a normal function allele or a no function allele, may have an increased likelihood of toxicity when treated with mercaptopurine as compared to patients with two normal function alleles. However, conflicting evidence has been reported. Other genetic and clinical factors may also influence mercaptopurine toxicity.