In contrast to the well-deWned nucleolar structures visible by EM, all the nucleolar proteins involved in ribosome biogenesis that have been examined, cycle between the nucleolus and the nucleoplasm in interphase cells. To summarize, it is now established that rapid diVusion of nucleolar proteins occurs in the nucleoplasm and recruitment to the nucleolus is permanent. Moreover, the diVerence in kinetics of several proteins shared between the nucleolus and the Cajal body suggests the existence of compartment-speciWc retention (Dundr et al. 2004 ).