Gimeracil (5-chloro-2, 4-dihydroxypyridine) is an orally active inhibitor against dihydropyrimidine dehydrogenase (DPYD), which degrades pyrimidine and the chemotherapeutic drug 5-fluorouracil (5FU). Gimeracil is shown to exhibit cancer-radiosensitizing efficacy in cultures in vitro (1 mM) and in mice in vivo (2.5-25 mg/kg, p.o.) in a DPYD-dependent manner via partial suppression of DNA repair by homologous recombination (HR).