In Drosophila imaginal wing discs, the achaete-scute (ac-sc) proneural genes and rhomboid (veinlet) are expressed in highly resolved patterns that prefigure the positions of sensory organs and wing veins, respectively. It is thought that these patterns are generated by a combination of factors (a prepattern) regulating these genes. We provide evidence for the existence of this prepattern by identifying two of its factors, Araucan and Caupolican. They are members of a novel family of homeoproteins, with homologs in vertebrates. Araucan and Caupolican, present in domains of the imaginal discs larger than those expressing ac-sc and rhomboid, are necessary for expression of these genes in the overlapping domains. Araucan and Caupolican appear to be positive, direct regulators of ac-sc.