Trastuzumab emtansine (T-DM1; Kadcyla®; Roche) is a first-in-class antibody–drug conjugate approved for the treatment of HER2-positive unresectable locally advanced BC (LABC) or metastatic BC (mBC) in patients previously treated with trastuzumab and a taxane, separately or in combination [7]. The gene discussed is ERBB2; the disease is breast cancer.