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HER2, also known as ErbB2 and Neu, is a 185-kDa transmembrane glycoprotein that is a member of the epidermal growth factor (EGF) receptor family of receptor tyrosine kinases. It has no ligand binding domain of its own and therefore cannot bind growth factors. However, it does bind tightly to other ligand-bound EGF receptor family members to form a heterodimer, stabilizing ligand binding and enhancing kinase-mediated activation of downstream signalling pathways, such as those involving mitogen-activated protein kinase and phosphatidylinositol-3 kinase. Amplification and/or overexpression of HER2 have been reported in numerous cancers, including breast and ovarian tumors. HER2 is a therapeutic target for the treatment of breast cancer and other carcinomas.