(D) DNA replication in U2OS cells expressing the E141A mutant (E141A) is suppressed. Cells were treated with H2O2 (2 mM in PBS, 5 minutes), MMS (1 mM in medium, 30 minutes) or mock (PBS, 30 minutes), followed by BrdU incorporation (30 μM in medium, 30 minutes). Representative images show BrdU-positive cells in cells expressing WT H2AX or E141A following genotoxic stress. Nuclei were stained with DAPI (blue). The bottom panel histogram shows the percentage of BrdU incorporation in each sample, Values are mean ± SD of three assays. 