The higher ranks of the class Actinobacteria were proposed and described in 1997.At each rank,the taxa were delineated from each other solely on the basis of 16S rRNA gene sequence phylogenefic clustering and taxon-specific 16S rRNA signature nucleotides.In the past 10 years,many novel members have been assigned to this class while,at the same time,some members have been reclassified.The new 16S rRNA gene sequence information and the changes in phylogenetic positions of some taxa influence decisions about which 16S rRNA nucleotides to define as taxon-specific.