vein1 (vn1) mutants lack portions of longitudinal wing vein 4 and the anterior crossvein. Stronger alleles, originally called defective dorsal discs, show vn is also required for the growth of the wing and haltere discs, as mutants for these alleles have tiny dorsal discs. vn functions nonautonomously and encodes a secreted EGF-like protein. Here we characterize the role of vn in the imaginal wing disc by describing the expression pattern and correlating this pattern with vn mutant phenotypes and the requirement for vn. vn is expressed in wing discs in a complex and dynamic pattern. In larval wing discs vn is first expressed in the presumptive notum and later in the wing-pouch and hinge regions. There is a striking localization of vn transcripts to intervein regions which begins with a stripe of expression straddling the AP boundary in late larval discs and develops in all intervein regions after puparium formation. We isolated new vn mutations including nulls and hypomorphs. Hypomorphic vn alleles revealed region-specific requirements for vn within the wing disc. We mapped lesions caused by 10 vn mutations and defined a minimum size of 48 kb for the gene. The phenotype and expression analyses show vn has an early role in global proliferation of the wing disc and specific roles in the development of the notum, hinge, longitudinal vein 4, and all intervein regions. The role of vn in the EGF receptor signaling pathway is discussed.