Non-protein-coding RNAs (ncRNAs) are expressed in viruses, archaea, prokaryotes and eukaryotes. They fulfil vital roles in the regulation of chromatin architecture, epigenetic memory, transcription, RNA splicing, editing and turnover. To date, at least 18 distinct types of ncRNA have been identified, which are classified according to their size, function, subcellular localization and target specificity.