Multiple acyl-CoA dehydrogenase (MAD) deficiency (glutaric aciduria type II) (OMIM 231680) can be caused by mutations in at least three of the genes involved in the mitochondrial ETF/ETF-QO complex (ETFA, ETFB and ETFDH), a pathway that transfers electrons from the first stage of β-oxidation to the electron transport system. Here, ETFB is linked to multiple acyl-CoA dehydrogenase deficiency.