16S ribosomal RNA (or 16S rRNA) is a component of the 30S small subunit of prokaryotic ribosomes. The genes coding for it are referred to as 16S rDNA and are used in reconstructing phylogenies, due to the slow rates of evolution of this region of the gene.  Carl Woese and George E. Fox were two of the people who pioneered the use of 16S rRNA in phylogenies.