RNomenclature.

RNAs that do not encode proteins, increasing evidence shows, are the rule rather than the exception. How do we call these RNAs? The term non-coding RNA should be rejected, we argue, since it constitutes a contradiction in terms: most if not all RNAs carry a code, even though that code may not specify an amino acid sequence. In naming these RNAs, we suggest to follow a natural distinction between two broad classes of RNAs. Class I RNAs are those that are transcribed but not translated, i.e., do not contain a translatable Open Reading Frame (ORF). Class II RNAs are transcribed and subsequently translated into amino acid sequences by the ribosomal translational apparatus. Class II RNAs comprise the familiar mRNAs, including peptide-coding RNAs. Class I RNAs, we suggest, are most fittingly called utRNAs (untranslated RNAs). The term npcRNAs (non-peptide/protein coding) can be used synonymously.