Fluorouracil is inactive as such in mammalian cells but is converted into the active 5-fluorodeoxyuridine monophosphate (FdUMP) by a variety of different metabolic pathways. The drug works by inhibiting the enzyme thymidylate kinase which results in reduced formation of thymidine and thus of DNA. The active metabolite FdUMP appears to form a stable complex with the folate cofactor N-5, 10-methylene tetrahydrofolate which inactivates thymidylate kinase. Fluorouracil as FdUMP is also incorporated into RNA which results in fluorination of RNA. The effect of fluorouracil on living cells is limited mainly to those in the proliferative phase but while cells in the G2 and S phase are most affected there may be effects at any stage of the cell cycle.