The present invention relates to a member of a novel gene family referred to as developmentally-regulated endothelial cell locus-1 (del-1). In particularly, the invention related to del-1 nucleotide sequences, Del-1 amino acid sequences, methods of expressing a functional gene product, and methods of using the gene and gene product. Since del-1 is expressed in endothelial cells and certain cancer cells, it may be useful as an endothelial cell and tumor marker. In addition, anti-Del-1 antibodies for immunological assessment of Del-1 protein expression as well as for inhibiting or stimulating Del-1 function are provided.