T-DM1, an antibody–drug conjugate (ADC) composed of trastuzumab and a derivative of maytansine as cytotoxic payload has been approved by the Food and Drug Administration (FDA) and the European Medicines Agency (EMA) in patients with HER2-positive metastatic disease after progression to trastuzumab and a taxane, and in the adjuvant treatment of patients with HER2-positive early breast cancer who have residual invasive disease after neoadjuvant taxane and trastuzumab-based treatment [21,51,52,53]. The gene discussed is ERBB2; the disease is metastatic neoplasm.