EGF Receptor (EGFR, ErbB1) is a transmembrane protein that exerts tyrosine kinase activity upon ligand induced activation. EGFR can be activated by binding EGF or at least six other structurally related protein ligands, including TGFα, HB-EGF, Betacellulin (BTC), Amphiregulin, Epiregulin, and Epigen. Upon activation, EGFR initiates a signaling cascade which includes dimerization and internalization, tyrosine phosphorylation, DNA synthesis of target genes, and, ultimately, cell proliferation. EGFR signaling plays a role in the growth and differentiation of normal cells, but elevated EGFR activity is correlated with the development and pathogenesis of certain cancers. Recombinant soluble human EGFR is a 621 amino acid glycoprotein comprising the extracellular domain of EGFR, and migrates at an apparent MW of 97.5 kDa by SDS-PAGE analysis under reducing conditions.