Human sDLL4 comprises the extracellular signaling domain of DLL, a member of a structurally-related family of single-pass type I trans-membrane proteins that serve as ligands for Notch receptors. DLL4 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 DLL4 gene in mice resulted in severe vascular defects and death before birth. Up-regulation of DLL4 expression has been implicated in the vascular development of certain tumors. Recombinant human sDLL4 is a 54.3 kDa glycoprotein containing 498 amino-acid residues.