TYMS and cancer: In cells, including cancer cells, 5-FU is converted by the actions of the enzymes, thymidine phosphorylase (TP) and thymidine kinase (TK) into 5-fluoro-2′-deoxyuridine-5′-mono-phosphate (FdUMP), to inhibit an enzyme called thymidylate synthase (TS) [11].