Expression of GFP:INCENP(FB) prevents breakage of dicentric chromosome bridges in cytokinesis. (A–C) Localization and mean intensity of Mre11 at the bridge DNA next to the midbody in HU- or TC-induced RPE-1 cells. (D–G) Localization and mean intensity of Aurora B (AurB) at the midbody ring in HU-, TC-, or TC+HU-induced cells. Mean ± SD from n cells. Values in HU-induced cells were set to 1. P < 0.001 (ANOVA and Student’s t test). (H–J) Localization and mean intensity of Aurora B at the midbody ring in TC-induced RPE-1 cells expressing GFP or GFP:INCENP(FB). Mean ± SD from n cells. Values in GFP:INCENP(FB) cells were set to 1. Broken chromatin bridges are indicated by dotted arrows. Arrowheads indicate DNA knots. Midbodies (boxed areas) were reexported and are shown at higher magnification. Scale bars, 5 μm. (K) Percentage of broken DNA bridges from cells treated as in H and I. Mean ± SD from three independent experiments (n > 120). ***, P < 0.001 (Student’s t test). Numbers below/next to each bar indicate n. (L) Proposed mechanism by which the abscission checkpoint senses chromatin bridges derived from catenated DNA in human cells (left). Dicentric chromosome bridges on the other hand, fail to activate the abscission checkpoint (right). Y, Top2α-DNA bond.