A and B, HR repair was analyzed in HR-reporter MCF7 cells treated with control siRNA (siCtrl) or siRNA targeting p53 (siP53) (A) or in MCF7 cells transfected by empty vector or vector encoding R273H mutant p53 (B). C and D, the surviving fraction of MCF7 siCtrl and siP53 cells (C) or MCF7 empty vector and R273 H mutant p53 expressing cells (D) was determined by clonogenic survival assay. Cells were first treated with 0 or 4 Gy IR and then treated 24h later with 0, 5, 7.5, or 10 µM ABT-888. Results are the average of three independent experiments.