Alcohol dehydrogenase 4 is an enzyme that in humans is encoded by the ADH4 gene. This gene encodes class II alcohol dehydrogenase 4 pi subunit, which is a member of the alcohol dehydrogenase family. Members of this enzyme family metabolize a wide variety of substrates, including ethanol, retinol, other aliphatic alcohols, hydroxysteroids, and lipid peroxidation products. Class II alcohol dehydrogenase is a homodimer composed of 2 pi subunits. It exhibits a high activity for oxidation of long-chain aliphatic alcohols and aromatic alcohols and is less sensitive to pyrazole. This gene is localized to chromosome 4 in the cluster of alcohol dehydrogenase genes. There is evidence that ancestors of modern humans developed the ADH4 producing gene some 10 million years ago. This probably helped them to take advantage of rotting fruit as they shifted to a terrestrial lifestyle.