Cell cycle distribution in the parent HeLa cells and the two radioresistant sub-lines 24 h after irradiation. Exposure of HeLa cells to a radiation dose of 4 Gy significantly increased the proportion of cells in the G2 phase of the cell cycle and decreased the proportion of cells in G1. The degree of G2 arrest was more apparent after irradiation with 16 Gy. The cell cycle distribution pattern of the radioresistant sub-lines also changed but without an increase in G2 cells after irradiation with 4 Gy. Increasing the irradiation doses to 16 Gy induced G2 arrest but the proportion of arrested cells was much lower than in the HeLa cell population.