To identify genes involved in the patterning of adult structures, Ga14-UAS (upstream activating site) technology was used to visualize patterns of gene expression directly in living flies. A large number of Ga14 insertion lines were generated and their expression patterns were studied. In addition to identifying several characterized developmental genes, the approach revealed previously unsuspected genetic subdivisions of the thorax, which may control the disposition of pattern elements. The boundary between two of these domains coincides with localized expression of the signaling molecule wingless.