Trastuzumab emtansine is a biological drug used in the treatment of human epidermal growth factor receptor-2 (HER2) positive metastasized breast cancer, formed by a monoclonal anti-HER2 antibody, trastuzumab, conjugated with a cytotoxin, emtansine, which is a tubulin-inhibitor. The gene discussed is ERBB2; the disease is breast cancer.