TYMS and cancer: The importance of targeting TS for 5-FU’s anti-cancer activity is underscored by its invariant clinical use in combination with folinic acid (Leucovorin; LV), a reduced folate co-factor that binds TS in a ternary complex with 5-Fluoro-2’-deoxyuridine-5’-O-monophosphate (FdUMP), the 5-FU metabolite that irreversibly inhibits TS enzymatic activity.