To be localized or retained within the nucleolus In eukaryotic cells, once imported or diVused into the nucleus, some proteins distribute throughout the nucleoplasm and others are targeted to speciWc nuclear compartments such as nucleoli. Proteomic analyses revealed that at least 700 proteins are localized in nucleoli (Andersen et al. 2002 (Andersen et al. , 2005 Leung et al. 2003) . Whereas the rules and signals that govern the nuclear localization and nuclear export of proteins are now well deWned, those concerning nucleolar localization are still debated.