(B) U2OS cells (WT) and U2OS H2AX knockout cells reconstituted with empty vector (H2AX KO+EV), wild type H2AX (H2AX KO+WT), or the E141A mutant (E141A) H2AX were treated with H2O2 (2 mM in PBS, 5 minutes) or MMS (1 mM in medium, 30 minutes), subsequently; the cells were cultured in fresh DMEM medium for 2 weeks. The numbers of colony formation were counted. Values are mean ± SD of three assays. 