The long term goal of this proposal is to understand how wing vein development is initiated in Drosophila wing imaginal discs. Five major veins run the length of the Drosophila wing. We have proposed that wing vein development is initiated at boundaries between distinct domains of cells situated along the anterior-posterior (A/P) axis of the wing imaginal disc in third instar larvae. In our convincing case for this hypothesis, we have provided a variety of evidence indicating that development of the second longitudinal vein (L2) is induced just anterior to a broad central domain of cells expressing a zinc finger transcription factor known as spalt- major (salm). We have proposed that salm expressing cells send a signal to adjacent anterior cells to activate expression of hormonereceptor family transcription factors encoded by genes at the knirpslknirps-related (kni/knrl) locus. The kni and knrl genes then function to organize gene expression in and around the L2 vein primordium. In this proposal we seek to understand the mechanism by which salm expressing cells activate kni and knrl expression in neighboring anterior cells by analyzing an enhancer element of the knilknrl locus which drives expression in the L2 primordium. We also propose to investigate the regulation of a second gene, abrupt (ab), which functions to organize formation of the L5 vein. In a key experiment, we propose to determine whether the kni~nrl and ab genes define specific vein types (i.e. L2 versus L5) or whether they function more generally in promoting vein versus intervein fates. These studies are of broad medical relevance to understanding the basis of birth defects. As defects in cell-cell signaling are involved in disease states such as cancer, the proposed studies also are relevant to such diseases since development of the L2 vein depends critically on cell-cell communication. In addition, the TGF-8 related signaling and EGF-R signaling pathways play prominent roles in maintaining and refining the vein pattern. Because mis-regulation of these known pathways causes aggressive forms of cancer, understanding the regulation of these pathways also is of significant relevance to cancer.