The transmembrane and secreted protein delta-like 1 homolog (DLK1, Protein delta homolog 1, Preadipocyte factor 1, Pref-1) belongs to the EGF-like family. It can be cleaved by a tumor necrosis factor-alpha converting enzyme (TACE) to release a soluble form. DLK1 plays important roles in regulating cell differentiation such as adipogenesis and osteogenesis. Membrane-bound DLK1 promotes hypertrophic myotube formation, whereas soluble DLK1 inhibits myotube formation. The soluble form of DLK1 inhibits preadipocyte differentiation, while membrane DLK1 represses preadipocyte proliferation. DLK1 is also associated with tumor invasion.