Trastuzumab (Herceptin®, Genentech/Roche, Inc.), a humanized monoclonal antibody directed against the extracellular domain of HER2, specifically acts on HER2 on cancer cell surfaces and has significantly improved patient prognosis, was approved for treatment of HER2-positive breast cancer and for the treatment of HER2-positive metastatic gastric or gastroesophageal junction adenocarcinoma (2). The gene discussed is ERBB2; the disease is gastroesophageal junction adenocarcinoma.