This proposal aims to contribute to an understanding of the organization of elementary genetic units in higher organisms and the regulation of the expression of such units in development. The rosy locus of chromosome 3 in Drosphila melanogaster carries the structural information for a peptide which, as a homodimer, functions as xanthine dehydrogenase in this organism. Mapping experiments have served to map the borders of the structural element, and variants which map outside and just to the left of the structural element appear to be associated with regulation (i.e., they are alterations in a cis acting control element. The direction of experiments in the current proposal are (1) mutagenesis experiments designed to make several classes of control variants, (2) analysis of such variants, and (3) examination of polygenic clustering about the rosy locus. BIBLIOGRAPHIC REFERENCES: Chovnick,A., M. McCarron, W.M. Gelbart and J. Pandey. 1975. Electrophoretic variants as a tool in the analysis of gene organization in higher organisms. In Ioszymes IV. Genetic and Evolution. Ed. C. Markert. Academic Press, New York. pp. 609-622. Duck, P. and A. Chovnick, 1975. Resolution of an equivocal genetic element in Drosphila melanogaster: organization of the maroon-like locus. Genetics 79:459-466.