Protein Delta Homolog 1 (DLK-1) is a transmembrane protein which contains a signal peptide, an extracellular domain with six tandem epidermai growth factor (EGF)-like domains, a single pass transmembrane domain, and a short cytoplasmic tail. It is found within the stromal cells in close contact to the vascular structure of placental villi, yolk sac, fetal liver, adrenal cortex and pancreas and in the beta cells of the islets of Langerhans in the adult pancreas. In addition, it is detected in some forms of neuroendocrine lung tumor tissue. DLK-1 may have a improtant role in neuroendocrine differentiation.