ACAD9 is a dual-function mitochondrial protein with roles in both fatty acid oxidation and Complex I assembly. As a fatty acid oxidation enzyme, ACAD9 catalyzes the first step of β-oxidation, converting long-chain acyl-CoA substrates to trans-2-enoyl-CoA using electron transfer flavoprotein as an electron acceptor 1. It plays a primary role in long-chain fatty acid metabolism particularly in the central nervous system 2. Beyond FAO, ACAD9 functions as a core component of the Mitochondrial Complex I Assembly (MCIA) complex, essential for respiratory Complex I biogenesis and oxidative phosphorylation 31. ECSIT binding induces conformational changes that switch ACAD9 from a FAO enzyme to a CI assembly factor independently of its catalytic FAO activity 4. ACAD9 deficiency causes combined impairment of both pathways: cardiac-specific knockout causes severe neonatal cardiomyopathy with mitochondrial dysfunction 5, while tissue-specific models reveal muscle weakness and metabolic crisis 6. The protein coordinates linoleic acid metabolism and redox homeostasis, with deficiency triggering oxidative phosphorylation failure and ferroptosis in lipid-rich environments 6. ACAD9 mutations cause mitochondrial complex I deficiency (nuclear type 20) and are associated with mitochondrial myopathies in childhood, with riboflavin showing therapeutic promise 7.