A: Impaired y-H2AX and 53BP1 accumulation in Ubc9 D100A cells. Stable U2OS cells were transfected with siRNA against endogenous Ubc9 for 72 h. Cells were treated with 125 µM H2O2 and immunofluorescence microscopy was performed. Left panel: representative immunofluorescence images. Right panel: quantification of cells with γ-H2AX and 53BP1 foci. Error bars represent S.E.M, n=3 independent experiments * p < 0.05. 