C: Phosphorylation of Chk2 is impaired in Ubc9 D100A. Experiment was performed as described in A. Stable U2OS cell populations were depleted from endogenous Ubc9 by siRNA for 72 h before addition of 250 µM H2O2. At the indicated times, cells were lysed in Laemmli buffer and analysed by immunoblotting with the indicated antibodies. 