(A) Clonogenic survival rate of HeLa cells treated with ZSTK474 (transient treatment; 24 h) after X‐irradiation. The cells were irradiated at the indicated doses of X‐ray, then 1 day later they were exposed to 1 μM ZSTK474 for 24 h. After changing the medium to ZSTK474‐free medium, the cells were incubated at 37°C for 2 weeks for analysis of their colony‐forming abilities. Open circles, control; closed circles, ZSTK474 treatment. Each point represents the mean survival fraction from three dishes. The graph is representative of three independent experiments. (B) Clonogenic survival rate of cells treated with ZSTK474 (continuous treatment; 14 days) after X‐irradiation. The cells were irradiated at the indicated doses of X‐ray, then 1 day later exposed to 1 μM ZSTK474 continuously. The cells were incubated at 37°C for 2 weeks for analysis of their colony‐forming abilities. Open circles, control; closed circles, ZSTK474 treatment. Each point represents the mean survival fraction from three dishes. The graph is representative of three independent experiments. (C) Microphotographs of colonies taken 2 weeks after treatment. Upper left, control; upper right, 6 Gy X‐rays only; lower left, 1 μM ZSTK474 only; lower right, combination of X‐rays and ZSTK474.