The anti-cancer activity of 5-FU, and its other forms floxuridine (FUDR) and capecitabine (Xeloda®), is largely attributed to the active metabolite, fluorodeoxyuridine-monophosphate (FUDR-MP or FdUMP), which inhibits the enzyme thymidylate synthase (TS), a critical enzyme in de novo nucleotide synthesis and cell survival [2]. The gene discussed is TYMS; the disease is cancer.