G2 arrest and its inhibition induced by low doses of X-rays. 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 Ser10-phosphorylated histone H3. KU55933 (10 μM), SB218078 (2.5 μM), or their solvent DMSO was administrated 30 min before IR. Mitotic cells were identified by strong fluorescence intensity of phosphorylated H3 signals. More than 5000 cells were scored at each dose and each treatment, and the results obtained from three independent experiments were pooled. Data indicate mean ± SD.