Expression of GFP:Nbs1 (ΔC20) reduces ATM localization to the midbody and induces chromatin bridge breakage in cytokinesis. (A–D and F–I) Localization of Nbs1, phospho-Aurora B-Ser331 (pAurora B-Ser331), ATM, or INCENP and mean intensity at the midbody in BE cells in cytokinesis with chromatin bridges. Values represent mean ± SD from n cells. (E) Western blot analysis of total Nbs1 and tubulin in BE cells expressing GFP:Nbs1 (ΔC20). (J) Frequency of BE cells with broken chromatin bridges. Values represent mean ± SD from three independent experiments (n > 150). ***, P < 0.001 (ANOVA and Student’s t test). (K) Treatment with mirin impairs ATM-Ser1981 phosphorylation (pATM-Ser1981) by etoposide. BE cells were treated with 10 µM etoposide in the absence or presence of 25 µM mirin for 4 h. (L–P) Localization of WT, Ser91A or Ser91D V5-INCENP in BE cells. Insets show 1.6× magnification of the midbodies. Broken chromatin bridges are indicated by dotted arrows. Scale bars, 5 µm.