The Pseudonocardiaceae are a family of bacteria in the order Actinomycetales and the only member of the suborder Pseudonocardineae. The species within the Pseudonocardiaceae family form a distinct clade in phylogenetic trees based on concatenated protein sequences. Additionally, Nakamurella multipartite, currently part of the order Frankiales, also formed a clade with the Pseudonocardiaceae species in 100% of the bootstrap replications of the phylogenetic trees. A conserved signature indel has been identified which is found in N. multipartite and all but one of the Pseudonocardiaceae species. This one-amino-acid insertion in UMP kinase serves to both provide a molecular marker for nearly all of the Pseudonocardiaceae and suggests N. multipartite is closely related to this group. Some evidence also suggests the orders Pseudonocardiales and Corynebacteriales are closely related. Several conserved signature indels have been identified which are found in both Pseudonocardiales and Corynebacteriales, including a three-amino-acid insertion in a conserved region of UDP-galactopyranose mutase. This insertion is also present in N. multipartite and Geodermatophilus obscurus, another member of Frankiales. Additionally, five conserved signature proteins have been identified which are found only in the Pseudonocardiales and Corynebacteriales orders. Homologs of the proteins are generally found in N. multipartite and G. obscurus, providing additional evidence of these two species being closely related to the orders Pseudonocardiales and Corynebacteriales. The phylogeny is based on 16S rRNA-based LTP release 123 by 'The All-Species Living Tree' Project. The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature and National Center for Biotechnology Information Notes:♠ Strains found at the National Center for Biotechnology Information, but not listed in the List of Prokaryotic names with Standing in Nomenclature