Trastuzumab is a humanized monoclonal antibody of the IgG1 subtype that binds to human epidermal growth factor receptor 2 (HER2), activating antibody-dependent cellular cytotoxicity and blocking HER2 signaling.1,2 It is considered a standard therapy for HER2-positive early and advanced breast cancers and advanced gastroesophageal adenocarcinoma.3-5. The gene discussed is ERBB2; the disease is breast carcinoma.