The nucleolus had consistently attracted the attention of investigators in the fields of cell biology and pathology. Because of its ubiquitous presence in the nucleus of eukaryotic cells, its rapid changes during their life cycle, and its rapid response to noxious agents, this organelle has been the subject of a large number of studies. Yet, the exact function and the very reason for the existence of the nucleolus (the only large cellular structure not delimited by a membrane) remain largely unknown. The ribosomes were discovered relatively late in the study of cells, but due to their crucial involvement in the protein synthesis machinery of all living