Adachi et al. reported that a second enzyme, uracil phosphoribosyltransferase (UPRT), which is absent from mammalian cells, directly converts 5-FU into 5-fluorouridine monophosphate which enhances the cytotoxicity of CD/5-FC gene therapy in an experimental malignant brain tumor, suggesting that co-expression of CD and UPRT genes have synergistic anti-tumor effects [52]. Here, UPRT is linked to brain cancer.