Clonogenic cell survival and cell cycle regulation after combination treatment of gefitinib and ionizing radiation (IR) in lung cancer cells. A. Clonogenic cell survival assays after gefitinib plus IR treatment. Cells were treated with gefitinib for 24 h, irradiated with graded doses of γ-radiation, incubated in the drug-containing media for another 48 h, and allowed to form colonies in drug-free medium. Surviving fractions for radiation+gefitinib were normalized by dividing by the surviving fraction for gefitinib alone. Closed circle, IR + vehicle (DMSO) treatment; Open circle, IR + 15 μmol/L gefitinib treatment. Error Bars, SE of three independent experiments in triplicate. **, P < 0.01; *, P < 0.05 compared with IR alone group. B, Cell cycle changes after gefitinib plus IR treatment. Cells were treated with 15 μmol/L gefitinib for 48 h and then irradiated. The cells were harvested after the indicated time points. Each cell cycle phase was measured by flow cytometry. Three independent experiments were performed and the results were all identical. Presented with a representative figure.