A human monoclonal antibody directed against Delta-like ligand-4 (DLL4) with potential antineoplastic activity. Anti-DLL4 monoclonal antibody REGN421 specifically binds to human DLL4, preventing its binding to Notch receptors and inhibiting Notch signaling, which may result in defective tumor vascularization and, so, the inhibition of tumor cell growth. DLL4 is the only Notch ligand selectively expressed on endothelial cells; DLL4/Notch signaling is required for the development of functional tumor blood vessels. Check for active clinical trials using this agent. (NCI Thesaurus)