TYMS and neoplasm: Mechanistically, the anti-tumor activity of 5-FU is considered to result from the formation of a ternary complex of 5-fluoro-2′-deoxyuridine-5′-monophosphate (metabolite of 5-FU), thymidylate synthase, and 5,10-methylenete terahydrofolate (metabolite of leucovorin).