What is 18S rRNA? 18S ribosomal RNA (18S rRNA) is a component of the small eukaryotic ribosomal subunit (40S), and 40S and 60S constitute eukaryotic ribosomes. As a structural RNA for eukaryotic ribosomes, 18S rRNA, the homologue of 16S rRNA in prokaryotes and mitochondria, is thus one of the essential components of all eukaryotic cells. […]