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.{{Cite journal | last1 = Schibler | first1 = U. | last2 = Wyler | first2 = T. | last3 = Hagenbuchle | first3 = O. | doi = 10.1016/0022-2836(75)90217-X | title = Changes in size and secondary structure of the ribosomal transcription unit during vertebrate evolution | journal = Journal of Molecular Biology | volume = 94 | issue = 3 | pages = 503–17 | year = 1975 | pmid = 809590}} ETS sequences characteristic to an organism can be used to trace its phylogeny.{{Cite journal |pmid = 10051397|year = 1998|last1 = Baldwin|first1 = B. G.|title = Phylogenetic utility of the external transcribed spacer (ETS) of 18S-26S rDNA: Congruence of ETS and ITS trees of Calycadenia (Compositae)|journal = Molecular Phylogenetics and Evolution|volume = 10|issue = 3|pages = 449–63|last2 = Markos|first2 = S.|doi = 10.1006/mpev.1998.0545}}