Activation and nuclear accumulation of cyclin B1–Cdk1 are coincident. (A) HeLa cells coexpressing the cyclin B1–Cdk1 activity FRET sensor and cyclin B1–mCherry were recorded at one image every 1 min 40 s by time-lapse fluorescence and differential interference contrast (DIC) microscopy as cells entered mitosis. Two cells entering mitosis are displayed for DIC (top), FRET efficiency determined by emission ratio (middle), and cyclin B1–mCherry (bottom). Bars, 10 µm. (B) Quantification of the nuclear accumulation of cyclin B1 and emission ratio in three different cells. The vertical dashed line indicates when signals started to increase.