Acad9 knockout mice [31]: • complete Acad9 inactivation leads to embryonic lethality; • cardiac-specific Acad9 knockout is associated with the lack of expression of Acadvl (encodes long-chain acyl-CoA dehydrogenase) and Ecsit (encodes a protein involved in complex I assembly) genes in cardiac tissue; expression of Acadm, encoding medium-chain acyl-CoA dehydrogenase, is reduced; complex I dysfunction; cardiomyopathy with atrial and ventricular thickening is diagnosed by day 14 of life; • muscle tissue-specific Acad9 knockout is associated with reduced exercise tolerance and lactic acidosis. Here, ACAD9 is linked to lactic acidosis.