The overwhelming majority of small nucleolar RNAs (snoRNAs) fall into two clearly defined classes characterized by distinctive secondary structures and sequence motifs. A small group of diverse ncRNAs, however, shares the hallmarks of one or both classes of snoRNAs but differs substantially from the norm in some respects. Here, we compile the available information on these excep- tional cases, conduct a thorough homol- ogy search throughout the available metazoan genomes, provide improved and expanded alignments, and investi- gate the evolutionary histories of these ncRNA families as well as their mutual relationships.

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