It was demonstrated that nucleolar proteins rapidly associate with and dissociate from nucleolar components in continuous exchanges with the nucleoplasm (Phair and Misteli 2000) . The recovery curve of GFP-Wbrillarin (DFC marker) in the nucleolus reached a plateau, 60 s after bleaching and the plateau indicated an immobile fraction of »15%. The diVusion coeYcient of Wbrillarin (estimated between 0.02 and 0.046 m 2 s ¡1 ) was 10 times lower in the nucleolus than in the nucleoplasm (Chen and Huang 2001; Phair and Misteli 2000; Snaar et al. 2000) . This value is proposed to reXect the time of residency of Wbrillarin engaged in nucleolar activity, and could explain the fact that the time of residency of Wbrillarin is shorter in the Cajal body than in the nucleolus (Dundr et al. 2004 ). The nucleolar proteins engaged in rRNA transcription and processing (respectively UBF, B23, Nop52, nucleolin and Rpp29) also move with rapid recovery rates in the nucleolus as does Wbrillarin (Chen and Huang 2001; Louvet et al. 2005) . Conversely the recovery rates of ribosomal proteins are slow (»3 times slower than nucleolar proteins). This could reXect a slower mechanism for ribosome protein assembly compared with transcription and processing (Chen and Huang 2001) , or alternatively, more stable associations of ribosomal proteins with the pre-rRNAs.