B: Left panel: The PP2A inhibitor okadaic acid rescues the y-H2AX defect of the "Ubc9 DA" cells. "Ubc9 wt" or "Ubc9 DA" cells were treated with 0.25 µM okadaic acid for 30 min. After addition of 250 µM H2O2 in fresh medium for 3 hours, immunofluorescence microscopy was performed as described in Fig. 7A. Error bars represent S.E.M, n=3 independent experiments. Right panel: Representative immunofluorescence images. 