B: H2O2 exposure leads to prolonged DNA damage in Ubc9 wt and Ubc9 D100A cells. Cells were treated as described in A. 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. An alkaline comet assay was performed to evaluate the DNA damage. Representative images are shown. Error intervals represent S.E.M, n=4 independent experiments. 