Trastuzumab is a fully-humanized monoclonal antibody, an antagonist of the HER2 receptor, and is indicated for the treatment of HER2-overexpressing breast cancer (both adjuvant and metastatic) and metastatic gastric or gastroesophageal junction adenocarcinoma [3,4]. The gene discussed is ERBB2; the disease is gastroesophageal junction adenocarcinoma.