The alcohol dehydrogenase locus (Adh) locus of Drosophila melanogaster contains 2721 base pairs with 4 exon coding regions. In a study of variation in a Drosophila populations, 43 nucleotide polymorphisms were found, but these led to only 1 variation in the protein, called the Adh-f and the Adh-s variants, which differ by only a single amino acid.