The dynamics of the processing nucleolar proteins was analyzed at the transition mitosis/interphase using rapid time-lapse video microscopy (Fig. 3) . The Wrst detectable assembly of proteins in foci occurred on the surface of the chromosome during telophase (Savino et al. 2001 ), followed by the progressive delivery of proteins to nucleoli ensured by progressive and sequential release of proteins from PNBs (Dundr et al. 2000) . Based on the observations of diVerent Wxed cells, it was concluded that the early processing proteins are recruited Wrst on transcription sites while the majority of the late processing proteins are still in PNBs (Fomproix et al. 1998; Savino et al. 1999 ). This sequence of events was conWrmed in living HeLa cells. Fibrillarin resides brieXy in PNBs (»15 min) before recruitment to the nucleolus, while Nop52 is maintained longer in PNBs (»80 min) (Savino et al. 2001) . The relative dynamics of early and late rRNA processing proteins at the time of PNB formation was examined using coexpression of GFP-Wbrillarin and DsRed-B23 (Angelier et al. 2005) . Once near the poles, 1-2 min after the onset of telophase, numerous bright Xuorescent foci containing both GFP-Wbrillarin and DsRed-B23 appeared almost simultaneously. For about 10 min, the relative amount of B23 in foci was Wve to six times higher than that of the dispersed proteins whereas the amount of Wbrillarin in the same foci was three to four times higher than that of dispersed proteins. Subsequently, Wbrillarin was released while B23 was still present in the foci. This clearly illustrates the presence of the two types of nucleolar processing proteins in the same PNBs and suggests diVerential sorting of these proteins. Conversely in the same conditions, simi-lar dynamics and Xows of GFP-Nop52 and DsRed-B23 were observed. Thus the processing proteins passed through the same PNBs and were released simultaneously suggesting that these proteins could form complexes in PNBs.