TYMS and cancer: 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.