The nuclear accumulation of cyclin B1–Cdk1 in prophase correlates with a change in nuclear transport machinery. (A and B) Cells coexpressing cyclin B1–mCherry and IBB-GFP (A) or GFP-M9 (B) were recorded as they entered mitosis (one image/30 s), and the import of cyclin B1 and export of IBB or M9 quantified on optical sections. (A) The mCherry (top) and GFP (bottom) images and mean curves of quantification of nuclear cyclin B1 and cytoplasmic IBB are shown. (B) mCherry (top), GFP (middle), and pseudo-colored GFP (rainbow look up table [LUT]; bottom) images and mean curves of quantification of the nuclear cyclin B1 and cytoplasmic M9 are displayed. Boxed images show the time point when cyclin B1 begins to enter the nucleus and GFP-M9 begins to exit. Error bars show SEM. Bars, 10 µm.