snoRNA Production snoRNAs are synthesised by two major mechanisms. The snoRNAs can either be directly transcribed by RNA polymerase II or produced through splicing. In the first case the snoRNA becomes capped like all other RNA polymerase II products. In the latter case, the snoRNAs are derived from introns that are spliced out during mRNA processing. The majority of the mRNAs known to harbour snoRNA introns code for proteins that are directly associated with protein synthesis, such as mRNAs for ribosomal proteins or initiation and elongation factors. In the most spectacular cases, the pre-mRNA only codes for snoRNAs. In these cases, the mRNA produced by splicing of the exons is degraded and the only stable product generated by the splicing process is the intron snoRNA.