5-fluorouracil is a nucleobase analogue that is uracil in which the hydrogen at position 5 is replaced by fluorine. It is an antineoplastic agent which acts as an antimetabolite - following conversion to the active deoxynucleotide, it inhibits DNA synthesis (by blocking the conversion of deoxyuridylic acid to thymidylic acid by the cellular enzyme thymidylate synthetase) and so slows tumour growth. It has a role as a xenobiotic, an environmental contaminant, a radiosensitizing agent, an antineoplastic agent, an immunosuppressive agent and an antimetabolite. It is a nucleobase analogue and an organofluorine compound. It is functionally related to a uracil.