Figure 3. A549-RR and H1299-RR cells showed cell cycle redistribution and reduced DNA damage response. (A) Cell cycle distribution was analyzed with flow cytometry. The percentage of cells at each phase of the cell cycle was quantified. (B) Cell cycle-related protein (p-CDK1, p21, p-Rb and CDC6) levels were measured using Western blot. (C) A549-RR, A549-control, H1299-RR and H1299-control cells were exposed to radiation at the same dose of 4 Gy and collected for comet assay as described in materials and methods. The percentage of DNA in the tail was calculated for 50 random cells. **P < 0.01, significant difference between radioresistant and control cells. (D) γH2AX, the indicator for DSBs, was detected by immunofluorescence staining. The results in (A) and (C) are the mean ± SEM of at least three independent experiments.