TYMS and neoplasm: The anti-tumor mechanism of 5-FU is associated with the metabolite, 5-F-deoxy-uridine monophosphate (dUMP), which inhibits thymidylate synthase (TS) activity and blocks the methylation of dUMP to deoxythymidine monophosphate (dTMP) which, in turn, affects DNA synthesis [25,26].