The 7.16.4 monoclonal antibody reacts with human and rat HER2 (human epidermal growth factor receptor 2) also known as neu peptide, CD340, ErbB-2, and p185. HER2 is a 185 kDa transmembrane, receptor-like glycoprotein with intrinsic tyrosine kinase activity that is part of several cell surface receptor complexes. HER2 lacks an identified ligand however, the kinase can be activated in the absence of a ligand when overexpressed. HER2 is a proto-oncoprotein that is commonly overexpressed on a variety of different tumors. Approximately 40% of human breast cancers overexpress HER2. HER2 overexpression is associated with poorer overall survival rates, shorter times to disease progression, and increased resistance to chemotherapy. Because of these clinical characteristics anti-HER2 monoclonal antibody therapy is now a standard for the treatment of advanced breast cancers that overexpress HER2. The 7.16.4 antibody has been shown to inhibit the growth of HER2-overexpressing tumors both in vitro and in vivo.