This phylum presently consists of a single class (Thermotogae), four orders (Thermotogales, Kosmotogales, Petrotogales, and Mesoaciditogales) and five families (Thermatogaceae, Fervidobacteriaceae, Kosmotogaceae, Petrotogaceae, and Mesoaciditogaceae). It contains a total of 15 genera and 52 species. In the 16S rRNA trees, the Thermotogae have been observed to branch with the Aquificae (another phylum comprising hyperthermophilic organisms) in close proximity to the archaeal-bacterial branch point. However, a close relationship of the Thermotogae to the Aquificae, and the deep branching of the latter group of species, is not supported by phylogenetic studies based upon other gene/protein sequences. and also by conserved signature indels in several highly conserved universal proteins. The Thermotogae have also been scrutinized for their supposedly profuse Lateral gene transfer with Archaeal organisms. However, recent studies based upon more robust methodologies suggest that incidence of LGT between Thermotogae and other groups including Archaea is not as high as suggested in earlier studies.