GTSE1 depletion causes defects in mitotic progression. (A) Select frames from live-cell imaging of HeLa cells stably expressing H2B-RFP and GFP-tubulin after transfection of either control or GTSE1 siRNA (see Videos 1, 2, and 3). For cells transfected with GTSE1 siRNA, examples of both chromosome alignment defects and multipolar spindles (middle) and chromosome alignment defects alone (bottom) are shown. Bars, 10 µm. (B) Quantification of mitotic phenotypes as assessed from live-cell imaging in A. Note that in GTSE1-depleted cells, only a small population (2%) underwent normal mitosis, whereas the majority of cells displayed both chromosome alignment defects and multipolar spindles (58% of cells) or chromosome alignment defects alone (40% of cells). As shown in the middle, most cells exhibited multipolar spindles after prolonged prometaphase arrest. Cells completing a bipolar anaphase within 3 h of NEB were classified as undergoing normal mitosis. Chromosome alignment defects and multipolar spindles were scored using DNA and microtubule morphology. Control n = 140 cells, and GTSE1 n = 100 cells, from two independent experiments.