Fluorouracil is a potent antitumor agent that interferes with DNA synthesis via noncompetitive inhibition of thymidylate synthase (TS). The interruption of this enzyme blocks the synthesis of pyrimidine thymidine, a nucleotide required for DNA replication. Fluorouracil Induces apoptosis in human primary and metastatic colon adenocarcinoma cell lines in vitro. It has also been shown to arrest the cell cycle at the G2 phase. Fluorouracil acts as an antimetabolite to uracil. Research in mice has shown that fluorouracil administration may lead to potential degeneration of the CNS by damage to oligodendrocytes responsible for production of myelin sheaths.