MADD is most commonly caused by pathogenic variants in electron transfer flavoprotein dehydrogenase (ETFDH), which encodes electron transfer flavoprotein-ubiquinone oxidoreductase (Etf-QO), a critical redox enzyme that transfers electrons from acyl-CoA dehydrogenases to the mitochondrial electron transport chain. The gene discussed is ETFDH; the disease is multiple acyl-CoA dehydrogenase deficiency.