HER2 (ERBB2: c-erb-b2) is an oncogene located on the long arm of chromosome 17 and is amplified in approximately 15% to 20% of breast cancers. Amplification or overexpression of HER2 is associated with shorter disease-free survival and poorer overall survival in breast cancer patients. Patients with HER2 gene amplification or overexpression are candidates for treatment with drugs that target the human epidermal growth factor receptor 2 (HER2) protein or its downstream pathways, such as trastuzumab (Herceptin) and pertuzumab.