The TPMT*9 allele is assigned as a no function allele by DPWG. Patients with the *9 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. Note that CPIC assigned TPMT*9 as an uncertain function allele. Other genetic and clinical factors may also influence mercaptopurine toxicity.