This family includes the highly conserved mitochondrial and bacterial proteins Sdh5/SDHAF2/SdhE. Both yeast and human Sdh5/SDHAF2 interact with the catalytic subunit of the succinate dehydrogenase (SDH) complex, a component of both the electron transport chain and the tricarboxylic acid cycle. Sdh5 is required for SDH-dependent respiration and for Sdh1 flavination (incorporation of the flavin adenine dinucleotide cofactor). Mutational inactivation of Sdh5 confers tumor susceptibility in humans. Bacterial homologs of Sdh5, termed SdhE, are functionally conserved being required for the flavinylation of SdhA and succinate dehydrogenase activity. Like Sdh5, SdhE interacts with SdhA. Furthermore, SdhE was characterized as a FAD co-factor chaperone that directly binds FAD to facilitate the flavinylation of SdhA. Phylogenetic analysis demonstrates that SdhE/Sdh5 proteins evolved only once in an ancestral alpha-proteobacteria prior to the evolution of the mitochondria and now remain in subsequent descendants including eukaryotic mitochondria and the alpha, beta and gamma proteobacteria. This family was previously annotated in Pfam as being a divergent TPR repeat but structural evidence has indicated this is not true. The E. coli protein, YgfY also acts as the antitoxin to the membrane-bound toxin family Cpta, pfam13166, whose E. coli member YgfX, expressed from the same operon as YgfY.