Approximately 15% to 25% of breast cancers (BCs) overexpress human epidermal growth factor 2 (HER2), which are historically aggressive, associated with poor prognosis and have a higher risk of relapse than HER2-negative cancers.[1–4] The humanized anti-HER2 monoclonal antibody trastuzumab was the first molecularly targeted agent to be approved by the FDA for the treatment of patients with HER2-positive BCs. The gene discussed is ERBB2; the disease is breast cancer.