Exit from mitosis in the presence of low concentrations of spindle MT poisons occurs from SAC satisfaction and not slippage. (A and B) Selected phase-contrast (top) and fluorescence (bottom) frames from video recordings of cyclin B/GFP-expressing RPE-1 cells as they enter and exit mitosis after no treatment (A) or in the presence of 50 nM nocodazole (B). Numbers define the time in minutes from NEB. (C) GFP intensity versus time plots reveal that the fluorescence intensity of control cells (e.g., no drug = cell shown in A) begins to rapidly decay at metaphase (18 min) after the SAC is satisfied and, once initiated, the chromatids disjoin ∼5 min later (23 min; arrow in C). A similar behavior is seen in cells treated with 50 nM nocodazole, except that the rapid decay of fluorescence intensity is delayed for a variable period during the prolonged mitosis (cell in B = cell 3). The cyclin B/GFP intensity in cells that enter and exit mitosis in 3.2 μM nocodazole exhibits a slow steady decline, with a slope reflecting the duration of mitosis, until the cell exits mitosis. Bars, 10 μm.