T-DM1 is an immuno-conjugate composed of trastuzumab, 1 humanized IgG1 monoclonal antibody and emtansine, a microtubule inhibitory cytotoxic agent derived from maytasin [5], linked by a thioether linkage, used in the treatment of HER2-positive breast cancer, as adjuvant management in invasive residual disease and locally advanced unresectable or metastatic disease; in both cases, patients must have received taxane treatment and anti-HER2-targeted therapy [6]. The gene discussed is ERBB2; the disease is breast cancer.