The analysis in living cells of intranuclear dynamics has recently become possible using Xuorescent fusion proteins. Time-lapse videomicroscopy can track the movement of large Xuorescent complexes in the cell, and Xuorescent recovery after photobleaching (FRAP) can measure the intracellular mobility or the residency time of Xuorescent proteins (Lippincott-Schwartz et al. 2001) . Inverse FRAP (iFRAP) quantiWes the loss of Xuorescence of the region of interest (ROI) after complete bleaching outside this region (Dundr et al. 2004 ). This constitutes a direct evaluation of the residency time of the proteins in the ROI. Another approach is the use of photoactivatable GFP (PA-GFP) to follow the traYc of the activated proteins (Patterson and Lippincott-Schwartz 2002) . This process is similar to pulsechase experiments since it makes it possible to follow a pool of labeled proteins starting at time zero. These technologies applied to nuclear dynamics have introduced new dimensions and unexpected concepts concerning nuclear functional compartmentation. The mobility of several GFPtagged nuclear proteins (nucleolar proteins, histones, DNA binding proteins, transcription factors, splicing factors, nuclear receptors) has been estimated by FRAP and the recovery of Xuorescence was slower than predicted for isolated diVusing proteins of similar size. FRAP recovery rates change with inhibition of transcription, decreased temperature and depletion of ATP indicating that recovery is correlated with nuclear activity.