ACAD9 belongs to a group of Acyl CoA dehydrogenases (ACADs). This family of proteins localize to the mitochondria and catalyze the rate-limiting step in the beta-oxidation of fatty acyl-CoA. One of the main energy producing metabolic pathways in eukaryotes is mitochondrial fatty acid beta oxidation. ACADs act on fatty acids containing 14 to 20 carbons. ACAD9 is specifically active toward palmitoyl-CoA and long-chain unsaturated substrates. Mutations in this gene cause acyl-CoA dehydrogenase family member type 9 deficiency. Alternate splicing results in multiple transcript variants.