Receptor tyrosine-protein kinase erbB-2, also known as HER2/Neu (human epidermal growth factor receptor 2), is a human epidermal growth factor receptor family member. Overexpression of HER2/Neu plays an essential role in developing and progressing specific subtypes of breast cancer. Approximately 15%-20% of all newly diagnosed breast cancers are HER2-positive, and HER2 testing identifies patients who benefit from HER2-targeted therapies, such as trastuzumab (Herceptin), lapatinib (Tykerb), pertuzumab (Perjeta), and T-DM1 (Kadcyla patients). There are two main ways to test HER2 status in cancer tissue: immunohistochemistry (IHC) which measures the amount of protein, and fluorescence in situ hybridization (FISH) which examines the number of HER2 gene copies.