– 5-Fluorouracil-13C,15N2 is intended for use as an internal standard for the quantification of 5-flurouracil by GC- or LC-MS. 5-Fluorouracil is a pyrimidine analog that irreversibly inhibits thymidylate synthase, blocking the synthesis of thymidine which is required for DNA synthesis. Intracellular metabolites of 5-fluorouracil exert cytotoxic effects by either inhibiting thymidylate synthetase, or through incorporation into RNA and DNA, ultimately initiating apoptosis.