Transforming growth factor (TGF-alpha, TGF-a, EGF-like TGF) is a polypeptide growth factor that shares significant structural and functional homology with epidermal growth factor (EGF). TGF is synthesized as a 160 amino acid transmembrane precursor molecule from which the mature 50 amino acid TGF alpha peptide is secreted following proteolytic cleavage by specific elastase-like enzymes. All the biological properties of TGF alpha and EGF are likely elicited by the binding of these ligands to the EGF receptor, therefore the biochemical cascade that follows activation of the EGF receptor tyrosine kinase is likely to be the same following activation of the receptor by either ligand.