Figure 5A. Representative images for fragile telomeres using the CO-FISH assay. Lagging strands (C-probe) and leading strands (G-probe) were marked with green PNA probes and red LNA probes, respectively. White circles and arrows indicate "smeary" or "doublet" telomeric signals, respectively. Quantification of fragile telomeres is shown on the right as the percentage of total telomeres counted (n=932-1073). Comparison of fragile to non-fragile telomere percentage was performed using Χ2-test. Data information: All scale bars: 5 μm. Data are shown as mean ± SD. B. Representative images for co-localization between RAD51 (green) and telomeres (red FISH probe). White arrows indicate representative RAD51-telomere co-localization events. Quantifications are shown on the right. A total of 120 nuclei were scored from two independent experiments. Data information: All scale bars: 5 μm. Data are shown as mean ± SD. B, the red line marks the mean value. Statistical analyses were performed with Student's two-tailed unpaired t-tests with Welch's correction for normality of distribution, unless specified otherwise. C. Representative images for TIF quantification: co-localization between 53BP1 (green) and telomeres (red FISH probe) in U2OS or IMRB cells with or without ectopic hTR overexpression. White arrows indicate TIF. Quantification of TIF is on the right. A total of 110-170 nuclei were scored from three independent experiments. Each dot represents the number of TIF per nucleus. Data information: All scale bars: 5 μm. Data are shown as mean ± SD. , C, the red line marks the mean value. Statistical analyses were performed with Student's two-tailed unpaired t-tests with Welch's correction for normality of distribution, unless specified otherwise. D. Representative images for TIF quantification, as in (A), in U2OS or U2OS-hTR cells, treated as indicated with DMSO or 5 μM ATRi or 5 μM ATMi for 20 h. White arrows indicate TIF. Data information: All scale bars: 5 μm. Data are shown as mean ± SD. E. Quantification of TIF from (D). At least 65 nuclei were scored in each of the two independent experiments performed. Each dot represents the number of TIF per nucleus. Statistical analyses were performed using one-way ANOVA with post-hoc Games-Howell's test for multiple comparisons. Data information: All scale bars: 5 μm. Data are shown as mean ± SD. the red line marks the mean value. Statistical analyses were performed with Student's two-tailed unpaired t-tests with Welch's correction for normality of distribution, unless specified otherwise. F. Representative images for co-localization between POLD3 (red) and RAP1 (green). White arrows indicate co-localization events. Quantification of co-localization events between POLD3 and RAP1 is shown on the right. A total of 190-230 nuclei were scored from three independent experiments. Data information: All scale bars: 5 μm. Data are shown as mean ± SD. F: the red line marks the mean value. Statistical analyses were performed with Student's two-tailed unpaired t-tests with Welch's correction for normality of distribution, unless specified otherwise. G. C-circle analysis using slot-blot assay hybridized with radioactive C-rich telomeric probe. Quantification is shown below (n=7). Statistical analyses were performed using the non-parametrical Mann Whitney test. Data information: All scale bars: 5 μm. Data are shown as mean ± SD. H,I. C-circle analyses in the indicated cell lines 72 h after treatment with siPOLD3 or siLuc used as control for transfection. Quantifications are shown below (n=3). siSMARCAL1 was used as positive control for increased C-circle production in cells with defective fork reversal activity (Zhang et al, 2019b). Data information: All scale bars: 5 μm. Data are shown as mean ± SD.