Many small RNA species associate with the nucleolar structure. Some of these small nucleolar RNAs (snoRNAs) are required for cleavage processing of ribosomal RNA precursors. There are many pseudouridine residues and methylated riboses in mature ribosomal RNA. For most, if not all, of these modifications, each site is selected by base pairing with a specific snoRNA species. Some snoRNAs are needed for the 2'-O-ribose methylation of at least one spliceosomal small nuclear RNA. Many snoRNAs, particularly in yeast, are generated from independent transcription units. Most vertebrate snoRNAs are produced by processing of introns from protein-coding transcripts. Some snoRNAs are made by processing of introns from non-protein-coding transcripts.