ERBB2 and breast cancer: Trastuzumab (Herceptin), a monoclonal immunoglobulin G1 (IgG1) that targets the internalizing HER2 receptor, has been used successfully in the treatment of HER2+ breast cancer and is the antibody component of a recently FDA-approved ADC therapy for the same indication, trastuzumab emtansine (Kadcyla)21, 22.