TYMS and cancer: One explanation is that supplementation of cancer cells with reduced folates in any form or quantity results in small expansion of 5–10 methylene tetrahydropteroylglutamate (CH2-H4PteGlu) pools4–11 up to concentrations far below that required to increase the tightness of binding of fluorodeoxyuridine monophosphate (FdUMP), the active metabolite of FUra, to thymidylate synthase (TS) for maximum stability of the ternary complex [FdUMP-TS-CH2-H4PteGlun] resulting in durable inhibition of the TS12–15 (Fig. 1).