ssDNA end formation at DSBs in NHEJ deficient cells. Cells 24 h after irradiation were treated in five different ways (A): 1. Cold DNA extraction; 2. Cold DNA extraction + 18 h at 50°C; 3. Cold DNA extraction + ExoVII + 18 h at 50°C; 4. Warm DNA extraction; 5. Warm DNA extraction + ExoVII. In (B) and (C) the relative amount of initial damage in the NHEJ deficient M059J cell line and the NHEJ proficient M059K cell line, respectively are shown after the different treatments outlined in A. The level of damage at 0 h with the cold DNA extraction protocol was set to 100%. The error bars represent the SD of three experiments (M059J) or the maximum deviations of two experiments (M059K).