Trastuzumab, as a humanized monoclonal antibody binding to domain IV of HER2, is approved by FDA for the treatment of HER2-positive breast and stomach adenocarcinoma/gastroesophageal junction adenocarcinoma cancer (https://www.cancer.gov/about-cancer/treatment/drugs/trastuzumab). The gene discussed is ERBB2; the disease is gastric adenocarcinoma.