Delta-like protein 4 (DLL4) is a type I transmembrane protein whose extracellular domain contains a DSL domain and eight tandem EGF repeats. After binding of its ligands Notch 1 and Notch 4, the extracellular and cytoplasmic domains are cleaved and the cytoplasmic domain is translocated into the nucleus. DLL4 is expressed in the vascular endothelium, neural epithelium, adrenal cortex, spleen, and lymph nodes. It is also highly expressed in some types of bladder, breast, and renal carcinomas. DLL4 is critical in vascular development and homeostasis.