ssDNA ends at DSBs in S-phase cells lacking DNA-PKcs. DNA-PKcs deficient cells (M059J) and DNA-PKcs inhibited cells (M059K+NU7026) were pulsed with 3H-thymidine 1 h before irradiation with 80 Gy. The size distributions of DNA fragments containing incorporated 3H-thymidine (i.e. DNA from cells in the S phase during the 3H incubation) in M059J and M059K+NU7026 cells at 4.1 h are shown in (A) and (B), respectively. The DNA was extracted by the warm or cold protocol. The amount of fragments < 1.1 Mbp was summed and the total fraction of S-phase DNA < 1.1 Mbp, extracted by the warm and cold protocol, in M059J and M059K+NU7026 was plotted for 4.1 h (C). Error bars represent the SEM from three experiments (M059J) or the maximum error from two experiments (M059K+NU7026).* Denotes a significant difference with the t-test (p < 0.025).