Distribution of the SOID values in G2 and mitotic cells. Exponentially-growing normal human primary fibroblasts were irradiated with 0.4 Gy of X-rays (A - D). Two hours after X-irradiation, cells were fixed and subjected to immunofluorescence staining for Ser139-phosphorylated H2AX and Ser10-phosphorylated H3. Cells were also exposed to 1.0 Gy of X-rays (E - H). They were fixed and subjected to immunofluorescence staining for Ser139-phosphorylated H2AX and Ser10-phosphorylated H3 at 2 hours (E and G), 6, 8 and 12 hours (F and H) later. The SOID values of phosphorylated H2AX foci in the control cells (A and C), 0.4 Gy-irradiated cells (B and D), and 1.0 Gy-irradiated cells (E - H) were measured. The SOID values obtained from 1.0 Gy-irradiated cells prepared at 6, 8 and 12 hours later were compiled in F and H. G2 cells (A, B, E and F) and mitotic cells (C, D, G and H) were identified as described in Figure 2. Dotted lines indicate 4000 SOID, and the numbers indicate the percentage of cells with the SOID above 4000.