Each eukaryotic cytosolic ribosome contains 4 molecules of RNA: 28S rRNA (25S rRNA in yeast), 5.8S rRNA, and 5S rRNA in the 60S subunit and 18S rRNA in the 40S subunit. The 18S rRNA, 5.8S rRNA, and 28S rRNA are produced by endonucleolytic and exonucleolytic processing of a single 47S precursor (pre-rRNA) (reviewed in Henras et al. 2015). Transcription of ribosomal RNA genes, processing of pre-rRNA, modification of nucleotide residues within the rRNA, and assembly of precursor 60S and 40S subunits occur predominantly in the nucleolus (reviewed in Hernandez-Verdun et al. 2010, Boschi-Muller and Motorin 2013), with a few late reactions occurring in the cytosol.