Dose-dependent induction of foci in G2 and mitotic cells. Exponentially-growing normal human primary fibroblasts were irradiated with 0, 0.02, 0.04, 0.08, 0.1, 0.2, and 0.4 Gy of X-rays, and two hours later, cells were fixed and subjected to immunofluorescence staining for Ser139-phosphorylated H2AX and Ser10-phosphorylated H3. Foci numbers of Ser139-phosphorylated histone H2AX in G2 cells and mitotic cells were counted. G2 cells (black bars) and mitotic cells (grey bars) were identified as described in Figure 2. Data indicate means ± SD.