DLL1 (Delta-like protein 1) is a transmembrane glycoprotein in the Delta/Serrate/Lag-2 (DSL) family of Notch ligands. It plays a role in central nervous system morphogenesis, intervertebral joint formation, inner ear hair cell development and patterning, and arteriogenesis. It regulates the homeostasis and differentiation of neural, pancreatic, and myogenic progenitor cells, T and B lymphocytes. DLL1 is overexpressed in a number of cancers and contributes to tumorigenesis. The extracellular region of DLL1 can be shed from the cell surface, and the intracellular domain can be cleaved by gamma-secretase to translocate to the nucleus and regulate gene transcription.