We focus specifically on two diseases: very long-chain acyl-CoA dehydrogenase deficiency (VLCADD) and malonyl-CoA synthetase deficiency (acyl-CoA synthetase family member 3 (ACSF3)) deficiency, which are both characterized by alterations in metabolic flexibility. Here, ACSF3 is linked to very long chain acyl-CoA dehydrogenase deficiency.