CtIP and exonuclease 1 protect cells from chromosomal damage. (A) At 72 h after transfection with the indicated siRNA oligonucleotides, U2OS cells were treated with either DMSO or camptothecin (1 h, 1 μM; acute treatment) and survival was determined by colony formation. Data represent the mean±s.e.m. of five independent experiments. (B) Cell survival at low doses of camptothecin from the data shown in (A). Data represent the mean±s.e.m. of five independent experiments. (C) Cells transfected as in (A) were treated with either DMSO or camptothecin for 24 h (chronic treatment) and survival was determined by colony formation. Data represent the mean±s.e.m. of three independent experiments. (D) Metaphase spreads from cells transfected and treated as described in (A) were analysed for chromosomal aberrations. A total of 50 metaphase spreads was analysed for each sample. The percentages of metaphase spreads displaying the indicated numbers of radial chromosomes are shown. CNTL, control; DMSO, dimethyl sulphoxide; EXO1, exonuclease 1; siRNA, small interfering RNA.