RAD51C accumulation at DNA lesions in S/G2 requires ATM and NBS1. (A) U2OS cells were irradiated (10 Gy of IR), fixed after 2 h, and stained with anti-RAD51C or anti-RPA (red) and anti–CENP-F antibodies (green). Nuclei were visualized with DAPI (blue). (B) U2OS cells were treated with ATM inhibitor (Ku55933) or solvent (DMSO) and then irradiated (10 Gy), fixed 2 h later, and stained with anti-RAD51C (red) and anti-RPA antibodies (green). (C) Atm-deficient cells stably transfected with a vector expressing full-length ATM or an empty vector were treated and processed as in B. (D) Nbs1-deficient cells stably transfected with a vector expressing full-length NBS1 or an empty vector were treated and stained as in B. (E) The percentage of nuclei exhibiting RAD51C or RPA foci was determined for at least 100 cells from D. The mean and standard deviation of three independent experiments are shown. Bars, 10 µm.