We propose that in the future, a better understanding of the complexity and variability of ribosome biogenesis will need to be established. For example, the diVerence between the information available in yeast and mammalian cells is of major importance. The diVerent steps of ribosome biogenesis and protein complexes are well characterized in yeast due to easy access of mutants. Similarly, Miller chromatin spreading for electron microscopy in yeast strains carrying mutations reveals the coupling of RNA pol I transcription with rRNA processing (Schneider et al. 2007 ). Additionally, the compaction into SSU processomes of pre-18S ribosomal RNA before cleavage was observed on Miller spreads (Osheim et al. 2004 ). There is presently no comparable information for the mammalian genes. Yet the tendency is to generalize and suppose that the information is similar in the two models. In the future, diVerences will most probably be revealed as well as the complexity of the regulation in diVerentiated cells. Along this line, it was demonstrated that basonuclin, a cell-type-speciWc rDNA regulator transcribes only one subset of rDNAs of a cell (Zhang et al. 2007b ). In such a diVerentiated cell, it remains to be established how the subset of rDNA repeats is selected.