The TPMT*1 allele has been assigned as a normal function allele by CPIC. Patients carrying the *1 allele in combination with another normal function allele may have decreased likelihood of experiencing toxicity when treated with azathioprine as compared to patients with two no function alleles or a no function allele in combination with a normal function allele. However, conflicting evidence has been reported. Other genetic and clinical factors may also influence azathioprine toxicity.