Receptor tyrosine-protein kinase erbB-2, also known as HER2/Neu (human epidermal growth factor receptor 2), is a member of the human epidermal growth factor receptor family. Overexpression of HER2/Neu plays an essential role in the development and progression of specific subtypes of breast cancer. Approximately 15%-20% of all newly diagnosed breast cancers are HER2-positive, and HER2 testing identifies patients who would benefit from HER2-targeted therapies, such as trastuzumab (Herceptin), lapatinib (Tykerb), pertuzumab (Perjeta), and T-DM1 (Kadcylapatients).