The short-chain dehydrogenases/reductases (SDR) are members of a very large family of enzymes, most of which are known to be NAD- or NADP-dependent oxidoreductases. Drosophila alcohol dehydrogenase was the first member of this family to be characterized. This family used to be called 'insect-type', or 'short-chain' alcohol dehydrogenases. Most dehydrogenases possess at least 2 domains, the first binding the coenzyme, often NAD, and the second binding the substrate. This latter domain determines the substrate specificity and contains amino acids involved in catalysis. Little sequence similarity has been found in the coenzyme binding domain although there is a large degree of structural similarity, and it has therefore been suggested that the structure of dehydrogenases has arisen through gene fusion of a common ancestral coenzyme nucleotide sequence with various substrate specific domains.