(A) 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 H2AX (E141A) were treated with H2O2 or MMS. Cell viability was measured after 24 hours using MTT assay. Values are mean ± SD of three assays. 