DNA combing, but not DNA spreading, can detect DNA single-strand gaps within individual sister chromatids. (A) Schematic depicting the use of pulse-labeling and FEN1 inhibitor (FEN1i) to detect single-strand gaps at unligated Okazaki fragments. (B) Dot plots of IdU/CldU ratios in cells treated ± FEN1i and ± S1 nuclease as indicated. Number of biological repeats N = 2. Bars represent the median values. At least 100 tracts were scored for each sample. Statistics: Kruskal–Wallis test. ns, non-significant, *P ≤ 0.05, ****P ≤ 0.0001.