TPMT (Thiopurine S-Methyltransferase): TPMT is an enzyme that metabolizes thiopurine drugs, such as azathioprine, mercaptopurine, and thioguanine, which are used in autoimmune diseases and cancer treatment. Genetic variations in TPMT affect enzyme activity, influencing drug breakdown and toxicity risk. Individuals with low TPMT activity are at higher risk of severe bone marrow suppression. Testing TPMT helps adjust thiopurine dosage to reduce adverse effects and improve treatment safety.