TPMT and Decreased total leukocyte count: However, the discovery of nudix hydrolase (NUDT15) as an important enzyme in the metabolism of thiopurine marks a significant breakthrough because its variants are highly prevalent in Asian populations and, as demonstrated in a Korean study, showed a predictive sensitivity of 89.4% for early leukopenia, compared to only 12.1% for TPMT variants [32], offering a more comprehensive explanation for interindividual variability, especially in populations where TPMT variants do not fully explain thiopurine intolerance [24,33,34,35,36,37].