Allelic deletions on chromosome 18q in Ͼ70% of primary colorectal tumors prompted the search for a tumor suppressor gene at that locus. An early result of this search was the cloning of a putative cell-surface receptor, DCC (deleted in colorectal carcinoma) (1). While the cloning of DCC brought much excitement that a tumor suppressor gene responsible for many colorectal cancers had been identified, 7 years later, the evidence that DCC is the putative tumor suppressor located on chromosome 18q remains inconclusive. Nonetheless, a role for DCC in tumor progression is suggested by the large number of different tumor types that have been reported to have lost DCC expression, including carcinomas of the pancreas, breast, prostate, bladder, and stomach; leukemias; neuroblastomas; and gliomas. The evidence for DCC as a tumor suppressor was examined in a recent review by Fearon (2).While the question of DCC as a tumor suppressor is still being debated, understanding of the normal physiological role of DCC has moved forward. Recent studies provide biochemical, functional, and genetic data suggesting that DCC is a receptor for the diffusible neural chemoattractant netrin-1 (3-5). Netrin-1 bound specifically to cells expressing DCC, and DCC mediated netrin-1-dependent outgrowth of commissural axons from dorsal spinal cord explants. This outgrowth was blocked by an anti-DCC mAb 1 that did not block the interaction between netrin-1 and DCC, suggesting that the interaction between netrin-1 and DCC may require additional factors. Furthermore, genetic analysis of UNC-40, a DCC homolog from Caenorhabditis elegans, suggests that there are several developmental functions attributed to UNC-40 that do not require netrin-1 (UNC-6). Taken together, these data suggest that while DCC/netrin-1 interactions are important, all the components for this guidance system have yet to be identified, and the functional role of DCC is not fully understood.A functional role of DCC in epithelial cells has also been suggested. Chuong et al. (6) showed that a Fab fragment of an anti-DCC mAb disrupted normal dermal condensation during feather bud formation in an embryonic chicken dorsal skin explant culture, a process that involves epithelial/mesenchymal cell interaction (6). In addition, the same mAb blocked aggregation of stage 34 (embryonic day 8) skin epithelial cells. These findings suggested that DCC participated in Ca 2ϩ -independent cell/cell interactions.DCC encodes a transmembrane protein with an extracellular domain composed of four Ig C2-like repeats, six fibronectin type III (FNIII)-like repeats, a single membrane-spanning region, and a 325-amino acid cytoplasmic domain (7). This complicated extracellular domain structure of DCC provides many candidate domains for mediating intracellular interactions. In this study, we set out to identify counter cell-surface DCC ligand(s) by using a DCC-Ig fusion protein, which bound to neural and epithelial derived cell lines. We have further demonstrated that the molecular basis for this interaction is ...