Lack of Plk1 results in a SAC-dependent mitotic arrest. (A) Representative images of Plk1(Δ/Δ) megakaryocytes in the absence or presence of the SAC inhibitor reversine. Spindles are shown by α-tubulin staining (green), whereas pH3 is shown in magenta. DAPI (blue) was used to stain DNA. Scale bars, 50 μm (or 20 μm in the 2 right columns). (B) Representative images of the indicated cultures in the absence or presence of reversine. Spindles are shown by α-tubulin staining (green), whereas pH3 is shown in magenta and DAPI (blue) was used to stain DNA. Scale bars, 50 μm (or 20 μm in the bottom row). (A-B) Arrows indicate curved nuclei around a single tubulin aster. (C) Mitotic index, as determined by detection of pH3, in Plk1- or Cdc20-deficient cells or control cells treated with Taxol or BI2536 in the absence or presence of the Mps1 inhibitor reversine. Data are mean ± SD. n = 50 cells per condition. **P < .01; ***P < .001; Student t test.