(A, C) p53−/− MEFs and (B, D) p53−/−Mdm2−/− MEFs were treated with Nutlin (10 µM) or vehicle control (DMSO). MEFs with ectopic Mdm2 overexpression served as a positive control. Following exposure to 5 Gy of γ-radiation, MEFs were fixed at intervals and immunofluorescence for γH2AX (A, B) and pS/TQ (C, D) was performed. The number of foci per cell was quantified. The mean of 2–3 independent experiments is graphed. Error bars represent SEM, and significance (*) determined using a confidence interval of 95%.