Mouse Pref-1 (preadipocyte factor 1), also known as Dlk-1 or FA1, belongs to the Notch/Delta/Serrate family of EGF-like repeat-containing proteins that act as ligands in the Notch signaling pathway (1-3). Mouse Pref-1 is synthesized as a 385 amino acid (aa) precursor consisting of a 23 aa signal sequence, a 282 aa extracellular region, a 24 aa transmembrane segment, and a 56 aa cytoplasmic tail. The extracellular region contains six tandem EGF-like repeats, a juxtamembrane region, and multiple sites for O- and N-glycosylation. Heterogeneous transmembrane forms of Pref-1, ranging from 50 to 60 kDa, can be attributed to variability in glycosylation (2-4). In addition to full-length Pref-1A, alternative splicing of the juxtamembrane and EGF-like repeat regions generates three major short forms, Pref-1B, -1C, and
-1D (5). Proteolytic cleavage at one of two extracellular sites in Pref-1A and 1B can produce two soluble forms: a 50 kDa large form and a 24-25 kDa small form (2, 5, 6). Processing of Pref-1C and Pref-1D, by contrast, can produce only small soluble forms. Only the large soluble form demonstrates biological activity (5). The ECD of mature mouse Pref-1 shares 94% and 83% aa sequence identity with the ECD of human and rat Pref-1, respectively. Pref-1 is widely expressed during embryogenesis (2). In adults, Pref-1 expression is restricted to preadipocytes, pancreatic beta  cells, thymic stromal cells, and cells of the adrenal and pituitary glands
(1, 7-9). Tumors of neuroendocrine origin have been found to express Pref-1 (10). Down-regulation of Pref-1 in preadipocytes is associated with differentiation into mature adipocytes (1-7). Pref-1 also regulates the differentiation of skeletal stem cells, thymocytes, and adrenal gland cells and inhibits hormone secretion in pituitary cells (1). Pref-1 has also been shown to inhibit preadipocyte proliferation (11, 12).