Growth factors and their receptors are involved in the regulation of cell proliferation and also play a key role in oncogenesis. In this study, a novel putative kinase receptor gene, termed eph, has been identified and characterized by molecular cloning. Its primary structure is similar to that of tyrosine kinase receptors thus far cloned and includes a cysteine-rich region in the extracellular domain. However, other features of the sequence distinguish the eph gene product from known receptors with tyrosine kinase activity. Thus the eph protein may define a new class of these molecules. The eph gene is overexpressed in several human carcinomas, suggesting that this gene may be involved in the neoplastic process of some tumors.