(A) Abra1-depleted or Rap80-depleted cells are sensitive to IR and UV. U2OS cells were treated with control oligos or siRNAs against Abra1, Rap80 or Brca1, incubated for 2 days, plated at low density, irradiated, and colonies counted after 14 days. (B) Analysis of the G2/M checkpoint. Cells were untreated or treated with 3 Gy IR as indicated, then incubated for 1 h at 37°C before fixation and p-H3 antibody staining. Three independent experiments were performed with siRNA oligos against Abra1. Two independent experiments were performed with siRNA oligos against Rap80 that yielded similar results. (C) Abra1-siRNA treated cells or Rap80-siRNA treated cells are defective for homologous recombination. DR-U2OS cells were transfected with siRNAs against luciferase, Brca1, Brca2, Abra1 or Rap80 separately. siRNAs against Brca1 or Brca2 were a mixture of three different siRNA oligos for each gene. Individual siRNA oligos were used for Abra1 or Rap80 genes. Three independent experiments were performed with siRNAs against luciferease, Brca1, Brca2 and Abra1.