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