For long, interest concerning the nucleolus was to establish how eYcient ribosome biogenesis occurs and the link of this function with the cell cycle. More recently the eVect of the disruption of ribosome biogenesis appeared very important when it was proposed that the nucleolus is a sensor of stress (Rubbi and Milner 2003) . Indeed disruption of ribosome biogenesis releases ribosomal proteins from the nucleolus that bind to MDM2 and inhibit p53 degradation (Lindstrom et al. 2007) . A connection between ribosomal stress and p53-dependent cell cycle arrest is now proposed (Gilkes et al. 2006) .