Relationship between the number and the SOID of foci per cell. Exponentially-growing normal human primary fibroblasts were irradiated with 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. The SOID values and the number of foci were examined in G2 cells. The SOID was measured as described in Methods. The SOID values and the corresponding foci numbers were plotted.