NUDT15 (Nudix Hydrolase 15) is an enzyme that plays a key role in the metabolism of thiopurine drugs, including azathioprine, mercaptopurine, and thioguanine. Genetic variations in NUDT15 can reduce its activity, increasing the risk of severe side effects such as bone marrow suppression. Testing for NUDT15 helps guide the appropriate thiopurine dosage to reduce toxicity and enhance treatment safety.