The "Betaproteobacteria" are one of the eight classes that make up the "Proteobacteria". The "Betaproteobacteria" are most closely related to the "Gammaproteobacteria", "Acidithiobacillia" and "Hydrogenophilalia", and together they make up a taxon which has previously been called ""Chromatibacteria"". Four orders of "Betaproteobacteria" are currently recognised — the "Burkholderiales", the "Neisseriales", the "Nitrosomonadales" and the "Rhodocyclales". The name ""Procabacteriales"" was also proposed for an order of endosymbionts of "Acanthamoeba", but since they cannot be grown in culture and studies have been limited, the name has never been validly or effectively published, and thus is no more than a nickname without any standing in nomenclature. An extensive reclassification of families and orders of the class based on a polyphasic analysis (including 16S rRNA gene analyses and 53-protein ribosomal protein concatamer analyses using the rMLST Multilocus sequence typing system) was published in 2017, that removed the order Hydrogenophilales from the class and into a novel class of the "Proteobacteria", the Hydrogenophilalia. The same study also merged the former order "Methylophilales" into the "Nitrosomonadales".