Purity >95%. 5-Fluorocytosine (5-FC), a fluorinated pyrimidine analog, is a synthetic antimycotic prodrug that is converted by cytosine deaminase to 5-fluorouracil. 5-fluorouracil, a widely used cytotoxic drug, is further metabolized to fluorinated ribo- and deoxyribonucleotides, resulting in the inhibition of DNA and protein synthesis, which has multiple effects including inhibition of Candida species and C. neoformans infections and cytotoxicity towards cancer cells. In combination with a retroviral replicating vector carrying a cytosine deaminase prodrug-activating gene, 5-FC has been shown to selectively eliminate CT26 and Tu-2449 tumor cells in vitro (IC50 s = 4.2 and 1.5 μM, respectively) and to significantly improve survival and reduce tumor size (at a dose of 500 mg/kg) in two different syngeneic mouse glioma models.