It has become apparent that nucleoli of diVerent cell types exhibit a variable number of FCs of diVerent sizes, with an inverse proportion between size and number (Hozak et al. 1989; Pébusque and Seïte 1981) . Generally cells with a high rate of ribosome biogenesis possess numerous small FCs. On the contrary, cells with greatly reduced metabolic and transcription activities, present small nucleoli with one large-sized FC such as in lymphocytes and in inactive mammalian neurons (Hozàk et al. 1994; Lafarga et al. 1989 ). In the more active neurons, one giant FC (GFC) of 1-2 m is observed together with small FCs (Fig. 1c, d) . It was demonstrated that the GFC is enriched in the upstream binding factor, the UBF transcription factor, in a small ubiquitin-like modiWer (SUMO)-1 and Ubc9 but lack ubiquitin-proteasome and 20S proteasome (Casafont et al. 2007 ). However, the possibility that only one FC might play a role in storage and become a GFC during intense nucleolar activity is still an open question.