NUDT15 (Nudix Hydrolase 15): NUDT15 is an enzyme involved in the metabolism of thiopurine drugs, such as azathioprine, mercaptopurine, and thioguanine. Genetic variations in NUDT15 can lead to reduced enzyme activity, increasing the risk of severe side effects like bone marrow suppression. Testing NUDT15 helps determine the appropriate thiopurine dosage to minimize toxicity and improve treatment safety.