CtIP-dependent DNA end resection is required for sustained checkpoint signaling. (A) U2OS cells were transfected for 2 d using the indicated siRNAs. Cells were irradiated (10 Gy), collected at the indicated time points, and processed as in Fig. 1 A. Numbers are as in Fig. 1 C. siUNC, Universal Negative Control siRNA. (B) U2OS cells were seeded on coverslips and siRNA transfected for 2 d. After IR, cells were fixed at the indicated times and stained for RPA and γH2AX. Pictures were acquired using confocal microscopy. Bar, 10 µm. (C) U2OS cells were transfected with control or CtIP siRNA and synchronized in G2 phase by double thymidine treatment. Cells were irradiated (15 Gy), collected at the indicated time points, and then processed for immunoblotting with the indicated antibodies. Numbers are as in Fig. 1 C. AS, asynchronized growing cells.