For instance, co‐treatment with leucovorin (5′‐formyltertrahydrofolate), which can facilitate the binding of FdUMP to TS, was shown to increase toxicity of 5‐FU in many cancer cell lines.7, 23 PRPP, a catalytic product of PRPS1, is the precursor of nucleotide biosynthesis and the important cofactor required for the conversion of 5‐FU to FUMP by OPRT. This evidence concerns the gene PRPS1 and cancer.