Dose-dependent induction of chromatid breaks. 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, mitotic cells were harvested and chromosome samples were prepared. Colcemid (0.1 μg/ml) was treated from immediately after IR to 2 hr after IR in order to collect metaphase cells. KU55933 (10 μM) was treated from 30 min before IR to 1.5 hr after IR, and washed out, then media were replaced by fresh media containing DMSO and Colcemid. After the media replacement, cells were cultured for additional 30 min, followed by metaphase harvest. More than 50 metaphases were examined per point. Black bars: control cells, Grey bars: KU55933-treated cells. ** indicates significant difference at p < 0.01.