Cyclin B1 nuclear import rate increases significantly at mitotic entry. Cells expressing either wild-type (WT) or 5xE cyclin B1–GFP were recorded, and the nuclear import of wild-type and 5xE cyclin B1 were quantified after treating G2 phase cells with 20 nM LMB to inhibit nuclear export. Note the sudden and strong increase of the wild-type cyclin B1 nuclear import rate when the cell enters mitosis.