Human sDLL-4 comprises the extracellular signaling domain of DLL, a member of a structurally-related family of single-pass type I transmembrane proteins that serve as ligands for Notch receptors. DLL-4 functions to specifically activate the Notch-1 and Notch-4 receptors. The Notch signaling pathway regulates endothelial cell differentiation, proliferation and apoptosis, and is essential for the development, maintenance and remodeling of the vascular system. Targeted deletion of the DLL-4 gene in mice resulted in severe vascular defects and death before birth. Up-regulation of DLL-4 expression has been implicated in the vascular development of certain tumors. The human DLL-4 gene consists of a 503 amino acid extracellular domain with one DSL domain, eight EGF-like repeats, a 21 amino acid transmembrane domain, and a 135 amino acid cytoplasmic domain. Recombinant Human sDLL-4 is a 54.3 kDa glycoprotein containing 498 amino acid residues.