The electron transfer flavoprotein (ETF) [<cite idref="PUB00004936"/>, <cite idref="PUB00005086"/>]serves as a specific electronacceptor for various mitochondrial dehydrogenases. ETF transfers electrons tothe main respiratory chain via ETF-ubiquinone oxidoreductase. ETF is anheterodimer that consist of an alpha and a beta subunit and which bind onemolecule of FAD per dimer. A similar system also exists in some bacteria.The beta subunit of ETF is a protein of about 28 Kd which is structurallyrelated to the bacterial nitrogen fixation protein fixA which could play arole in a redox process and feed electrons to ferredoxin.The beta subunit protein is distantly related to and forms aheterodimer with the alpha subunit <db_xref db="INTERPRO" dbkey="IPR001308"/>.