Protransforming Growth Factor-alpha (TGF-alpha), also known as sarcoma growth factor, TGF-type I and ETGF, is a member of the EGF family of cytokines. It is expressed in monocytes, brain cells, keratinocytes and various tumor cells. ProTGF-alpha signals through EGFR and acts synergistically with TGF-beta to promote the proliferation of a wide range of epidermal and epithelial cells. It may function as either a membrane-bound ligand or a soluble ligand. Membrane-bound proTGF-alpha plays a role in cell-cell adhesion and juxtacrine stimulation of adjacent cells. The soluble form of the cytokine is released from the membrane-bound form by proteolytic cleavage and acts as a mitogen for cell proliferation.