Transforming Growth Factor alpha (TGFα) is a 6 kDa polypeptide mitogen. Its precursor protein is a 160 amino acid integral membrane glycoprotein that is cleaved by a protease to produce the 50 amino acid mature TGFα. TGFα is produced by keratinocytes, macrophages, hepatocytes, and platelets; its synthesis is stimulated by viral infection. In mammary tissue, synthesis is stimulated by estrogen. Abundant TGFα expression is seen in transformed cells and tumors. It is expressed in modest amounts in nontransformed cells during development of mammalian embryos. TGFα is a ligand for EGFR and activates the EGFR signaling pathway promoting tumor progression by triggering downstream signaling molecules like AKT and MAPK.