5-FluoroUracil (5FU) is a thymine
analogue. It has a fluorine atom where thymine has a methyl group.
It is used as an anti-cancer drug, especially against colon cancer.
It acts as a selective inhibitor of the enzyme thymidylate synthase (TS). This prevents thymidine phosphate (as dTTP) from being incorporated into DNA which dividing cells, especially cancer cells, need for growth.