synchronization of NHEJ deficient cells prevents the formation of ssDNA at DSBs. (A) G1-synchronized V3 (DNA-PKcs deficient) and (B) GM16147 (XRCC4 deficient) cells were irradiated and at different repair times the DNA was extracted with the warm or cold protocol. The number of DNA fragments was determined by PFGE and the number of DNA fragments detected at 0 h by the cold protocol was set to 100%. The corresponding remaining damage at 4 h in asynchronous cells is plotted for comparison. (C) The V3 and (D) GM16147 cell cycle distributions at the time of irradiation. The corresponding distributions in the asynchronous cell population are seen in the insert. Error bars represent the SD of three experiments.