Delta-like protein 1 (DLL1) is a human homolog of the notch delta ligand and is a member of the delta/serrate/jagged family. It activates notch receptors and plays a major role in cell-to-cell communication and differentiation in many tissues. DLL1 mediates cell fate decisions during hematopoiesis and differentiation into T cells and NK cell precursors. Notch signaling also plays a role in neuronal function and development, cardiovascular formation and morphogenesis, and endocrine development. Notch signaling is dysregulated in many cancers.