In Human Epidermal growth factor Receptor 2 (HER-2) positive breast cancer, monoclonal antibodies that inhibit HER2 (Trastuzumab) [67] or disrupt HER2 dimerization (Pertuzumab) [70] as well as antibody-drug conjugate that delivers chemotherapy directly to HER2-positive cancer cells (Ado-trastuzumab emtansine) [71], are commonly used for improving efficacy while reducing systemic toxicity. The gene discussed is ERBB2; the disease is cancer.