Cells lacking the abscission checkpoint exhibit elevated genome damage and are sensitized to replication stress. (A) HCT116 cells with (WT) or without (δCHMP4C) endogenous CHMP4C expressing the indicated HA-CHMP4C construct were stained for 53BP1 (green), α-tubulin (red), and DNA (DAPI, blue). (Scale bars, 20 μm.) Insets show gray-scale images of boxed cells. (B, Upper) Numbers of 53BP1 foci per cell were determined from three independent experiments and binned into the designated categories. Shown are the mean ± SD from >900 cells. P values were calculated using two-way ANOVA and Sidak’s multiple comparisons test comparing each category to control (WT); *: 0–2 foci; #: more than six foci; */#P < 0.05; **/## = P < 0.005; ***/### = P < 0.001. (Lower) Representative immunoblots from B, Upper. (C) Cells stably expressing the indicated HA-CHMP4C construct were treated with the indicated siRNA for 48 h and then with DMSO or 30 nM aphidicolin for 24 h. The number of cells connected by midbodies was scored. (D) HCT116WT or HCT116δCHMP4C cells expressing the indicated HA-CHMP4C constructs were cultured in the continuous presence of DMSO (blue trace) or 30 nM aphidicolin (red trace), and cell numbers were determined at the indicated time points. Plotted are the mean ± SD from three independent experiments. See also SI Appendix, Fig. S7A.