Pref-1 (preadipocyte factor 1), also known as deltalike 1 (DLK1) and fetal antigen 1 (FA1), is a member of the Notch/Delta/Serrate protein family. It is a transmembrane protein that can be proteolyically cleaved to generate soluble forms. Pref-1 is believed to be a co-ligand for Notch signaling, enhancing or inhibiting Notch activation following binding of a ligand. It is expressed in numerous endocrine cells, and is highly expressed in preadipocytes, but not in mature adipocytes. In adipose tissue, Pref-1 inhibits adipogenesis and is often used as a preadipocyte marker.