Purpose: To find a new molecule that affects p53-dependent radiosensitivity.Methods and Materials: A mouse sarcoma cell line, QRsP(p53+/+), was used. From this cell line, we established a radiosensitive clone and a radioresistant one. Colony assay, p53 gene transfer, a luciferase assay for p53 and p21, animal transplantation experiment, and DNA array analyses were performed.Results: Microarray showed marked reduction of a transcription factor, ATF5, both in vitro and in vivo for the radiosensitive clone. Interestingly, flow cytometric analysis demonstrated marked apoptosis for the radiosensitive clone by p53 cloned adenovirus infection. Luciferase reporter assay revealed that ATF5 suppressed the transactivational activity of p53 and p63. By ATF5 gene transfer, the radiosensitive clone regained resistance to both ionizing-radiation and Ad-p53 infection-induced cell death. Surprisingly, time-lapse cell migration observation revealed greater cell motility for ATF5-transfected radiosensitive clone.Conclusions: It seems likely that ATF5 is a potent repressor of p53 and elevated expression of ATF5 in a tumor may relate to enhanced malignant phenotypes, such as radioresistance or greater cell motility.