RPE-1 cells were arrested in early mitosis by inhibiting Haspin, washed, and released into control (pH 7.4) or acidic (pH 6.4) medium without inhibitor. The nuclear pH gradient was dissipated with nigericin. Mitosis was allowed to proceed for 10 minutes, cells were fixed and labeled for DIDO (all panels, green) and H3K4me3 (panel a, red) or ING1 (panel b, red). DNA was counterstained with DAPI (all panels, blue). Color separations are shown in Supplementary Figures S1 and S2. Scale bars, 10 μm.