EGFR and cancer: The search for cell surface tumor markers has identified many proteins that are enriched in cancer cells versus normal cells, including integrins, epidermal growth factor receptor, G protein-coupled receptors, and other various receptors such as gonadotropin-releasing hormone receptor, vasoactive intestinal peptide receptors 1 and 2, neurotensin receptor 1, aminopeptidase N, and keratin 1, which are overexpressed in breast cancer [47,52,53,54,55,56,57].