External transcribed spacer

External transcribed spacer (ETS) refers to a piece of non-functional RNA, closely related to the internal transcribed spacer, which is situated outside structural ribosomal RNAs (rRNA) on a common precursor transcript.^[1] ETS sequences characteristic to an organism can be used to trace its phylogeny.^[2]

Organization of the eukaryotic nuclear ribosomal DNA tandem repeats.