TYMS and neoplasm: The following published evidence led us to measure gemcitabine metabolites in plasma and tumor (Fig. 7B): first, that the gemcitabine metabolite 2′,2′-difluorodeoxyuridine can inhibit thymidylate synthase (9), and second, that the gemcitabine metabolites 2′,2′-difluorodeoxycytidine and 2′,2′-difluorodeoxyuridine can compete with 18F-FLT by sharing some nucleoside metabolic enzymes and transporters (11).