Telomeric 53BP1 foci in RECQL4 knockdown U2OS cells. A, Western blot showing the level of RECQL4 in scrambled shRNA-treated (Scramble) and RECQL4-targeted shRNA-treated (RECQL4 KD) U2OS cells. Bands corresponding to RECQL4 and actin are shown by arrows. B, confocal microscopic images of representative cells showing TRF1 (red) and telomere (green) signals in scrambled shRNA-treated (panels 1 and 2, Scramble) and RECQL4 shRNA-treated (panels 4 and 5, RECQL4 KD) U2OS cells. Colocalized foci are visible as yellow dots in panels 3 and 6. Nuclear staining (with DAPI) is shown in the merged images. Scale bar = 10 μm. C, average numbers of TRF1 signals/cell in scrambled and RECQL4 KD U2OS cells. The error bars represent mean ± S.D., n = 50. D, confocal microscopic images showing colocalization of 53BP1 foci (green) and TRF1 (red) signals in scrambled (panels 1–3) and RECQL4 KD U2OS cells (panels 4–6) and in aphidicolin-treated scrambled U2OS cells (panels 7–9). Nuclear staining with DAPI is shown in the merged image. Some of the colocalized foci are marked with white arrows. Close-up images of some of the colocalized foci are shown next to panel 6. Scale bar = 5 μm. E, histograms showing the frequency distribution of telomeric 53BP1 foci (TIF) in scrambled, RECQL4 KD, and aphidicolin-treated scrambled U2OS cells (n = 70).