In the previous study we have detected the nuclear gH2AX/53BP1/ATM foci even after one or two months culture of in vitro X-irradiated normal human diploid fibroblasts (NHDFs). The persistence of such nuclear foci appears to be attributable to the existence of unrepairable double strand breaks (DSB) in non-apoptotic cells. We here report that the generation of such unrepairable foci is strictly dose dependent but not dependent on growth condition; equivalent frequency of unrepairable foci production was observed in quiescent and serum stimulated cells if the radiation doses were the same. The unrepairable foci were larger than those of vast majority of repairable foci, which were generated immediately after irradiation. However, the unrepairable foci still contained proteins of gH2AX, 53BP1, phosphorylated ATM, as well as they retained polyubiqutinated forms of chromatin components and BRCA1, indicating that repair and checkpoint factors were kept activated at the foci.