5-Fluorouracil is a potent antitumor agent that affects pyrimidine synthesis by inhibiting thymidylate synthetase thus depleting intracellular dTTP pools. 5-Fluorouracil is metabolized to ribonucleotides and deoxyribonucleotides, which can be incorporated into RNA and DNA. Treatment of cells with 5-Fluorouracil leads to an accumulation of cells in S-phase and has been shown to induce p53 dependent apoptosis.