DPYD (Dihydropyrimidine Dehydrogenase): DPYD is an enzyme that breaks down fluoropyrimidine drugs, such as 5-fluorouracil (5-FU) and capecitabine, widely used in cancer treatment. Genetic variations in DPYD can reduce enzyme activity, increasing the risk of severe toxicity, including bone marrow suppression and gastrointestinal side effects. Testing for DPYD helps identify patients who need dose adjustments or alternative therapies to enhance safety and efficacy.