The axonal projection mutations irregular chiasm C of Drosophila melanogaster co-map and genetically interact with the roughest locus required for programmed cell death in the developing retina. We have cloned the genomic region in 3C5 by transposon tagging and identified a spatially and temporally highly regulated single transcription unit that produces a major mRNA species of approximately 5.0 kb. Postembryonic expression is strong in the developing optic lobe and in the eye imaginal disc. The gene encodes a transmembrane protein (764 aa) with five extracellular immunoglobulin-like domains and similarity to the chicken axonal surface glycoprotein DM-GRASP/SC1/BEN. Both known irreC alleles reduce the level of transcription, while the roughestCT mutation disrupts the intracellular domain of the protein.