The ECI1 gene encodes 3,2-trans-enoyl-CoA isomerase, a mitochondrial enzyme belonging to the hydratase/isomerase superfamily of proteins. This enzyme plays a key role in the beta-oxidation of unsaturated fatty acids. Specifically, it catalyzes the shift of the double bonds of 3-cis- and 3-trans-isomers to the 2-trans-enoyl-CoAs. By doing so, the enzyme allows the re-entry of the 3-cis- and 3-trans-enoyl-CoA esters into the beta-oxidation spiral.