Aim: To investigate the effect of the Tobl gene, a member of the Transducing Molecule of ErbB2/B-cell Translocation Ggene (TOB/BTG) family, by using the adenovirus-mediated expression of Tob 1 on radiosensitivity in a human breast cancer cell line MDA-MB-231. Methods: Cell survival was determined by clonogenic assay. Apoptosis was evaluated by DNA fragmentation gel electro-phoresis and terminal deoxynucleotidyl transferase-mediated nick end labeling assay. Protein expression was analyzed by Weste blot assay and DNA repair was measured by a host cell reactivation assay. Results: We demonstrated that pre-irradiation treatment with Ad5-Tob 1 significantly increased radiosensitivity,accompanying the increased induction of apoptosis and the repression of DNA damage repair. Furthermore, Ad5-Tob 1-mediated radiosensitivity correlates with the upregulation of the pro-apoptotic protein Bax and the downregulation of several DNA double strand break repair proteins, including DNA-dependent protein kinases, Ku70 and Ku80, and X-ray-sensitive complementation group 4.Conclusion: Tobl, as a new radiosensitizer, is a new target in the radiotherapy of breast cancer via increasing apoptosis and suppressing DNA repair.