The epidermal growth factor receptor (EGFR; ErbB-1; HER1 in humans) is a member of the ErbB family of receptors, a subfamily of four closely related receptor tyrosine kinases: EGFR (ErbB-1), HER2/neu (ErbB-2), Her 3 (ErbB-3) and Her 4 (ErbB-4). EGFR is activated by binding of its specific ligands, including epidermal growth factor (EGF) and transforming growth factor alpha (TGF-α). Upon activation by its growth factor ligands, EGFR undergoes a transition from an inactive monomeric form to an active homodimer, which stimulates its intrinsic intracellular protein-tyrosine kinase activity. Deficient signaling of the EGFR and other receptor tyrosine kinases in humans is associated with diseases such as Alzheimer's, while over-expression is associated with the development of a wide variety of tumors.