Past studies showed that molecular targeted therapies against HER2 amplification improved prognosis of HER2 positive breast and gastric cancer patients.[6] Gene amplification and overexpression of this protein are seen in approximately 20% to 30% of breast cancer cases.[7] Trastuzumab, the first HER2-directed monoclonal antibody, is a milestone in the treatment of HER2-positive breast cancer at all stages of the disease and in all treatment lines.[8] Four anti-HER2 agents have been currently used for the treatment of breast cancer in Japan. The gene discussed is ERBB2; the disease is breast cancer.