Abstract
Two widely separated loci control xanthine dehydrogenase activity in Drosophila melanogaster. Extracts of mutants at each locus do not contain detectable amounts of this enzyme. Incubation of mixtures prepared by combining extracts from the mutants of each locus produces enzyme activity. The available data can be interpreted in terms of either a "combining-subunits" or a "catalvticactivator" hypothesis.