The human epidermal growth factor receptor (HER2) is overexpressed on 15–20% of breast cancers (BC)1 and is the therapeutic target for trastuzumab (Herceptin, Roche), pertuzumab (Perjeta, Roche) and the antibody-drug conjugate (ADC) trastuzumab-emtansine (T-DM1; Kadcyla, Roche).2 Here, ERBB2 is linked to breast cancer.