Electron transferring flavoproteins (ETFs) are heterodimeric FAD-containing proteins that function primarily as soluble electron carriers between various flavoprotein dehydrogenases to membrane-bound ETF ubiquinone oxidoreductase, which reduces ubiquinone to ubiquinol which enters the respiratory chain. (Adapted from PMID:17941859). Electron transfer flavoprotein complex is an FAD-containing heterodimer that functions as a soluble electron carrier between various flavoprotein dehydrogenases to membrane-bound ETF ubiquinone oxidoreductase (Etf-QO), which reduces ubiquinone to ubiquinol which enters the mitochondrial respiratory chain. (Adapted from PMID:17941859.)