Cyclin B1 but not Cdc25C moves into the nucleus before the nuclear pores become permeable to tubulin. (A and B) Cells coexpressing cyclin B1–GFP and mCherry–α-tubulin (A) or cyclin B1–mCherry and GFP-Cdc25C (B) were recorded as they entered mitosis (one image/40 s and one image/30 s, respectively), and the import of cyclin B1, α-tubulin, and Cdc25C was quantified on optical sections. (A) The GFP (top) and mCherry (bottom) images and mean curves of quantification of nuclear cyclin B1 and α-tubulin are shown. (B) mCherry (top) and GFP (bottom) images and mean curves of quantification of the nuclear cyclin B1 and Cdc25C are displayed. Error bars show SEM. Bars, 10 µm.