Eg5 inhibition counteracts the induction of spindle pole fragmentation by Aurora-A inactivation. The protocol to inhibit Aurora-A by MLN8237 in cells progressing towards mitosis is depicted (time intervals not represented to scale). Control cultures were treated with solvent (DMSO) in the same time window. When indicated, MON was added 1 hour before harvesting. Note the absence of active phosphorylated (pThr288) Aurora-A (in red in IF panels) in cells treated with MLN8237. Upper histograms represent the percentage of all spindle and MT abnormalities in control and MLN8237-treated cultures (200 counted PM/M per condition in 2 experiments); the grey fraction of the histograms represents mitoses with spindle extrapoles, while other defects (monopolar or disorganised spindles, few and short MTs) are in white. Lower histograms and IF panels show that concomitant Eg5 inhibition by MON prevents MLN8237-induced spindle pole fragmentation (note the failure of centrosome migration reflecting Eg5 inactivation in lower IF panels). 200 PM/M per condition were counted in 2 experiments. Error bars represent s.d. **: p < 0.001, χ2 test. Red asterisks indicate significant differences with respect to DMSO controls, and black asterisks significant differences between Aurora-Ai mitoses with active or inactive Eg5. Scale bar: 10 μm.