ERBB2 and breast cancer: Trastuzumab (Herceptin®), an immunoglobulin G1 (IgG1) antibody consisting of two mouse-derived antigen binding sites specific to the HER2 receptor extracellular domain (ED) and a humanized Fc portion (9), has been hailed as one of the successes of personalized medicine for the treatment of HER2-positive breast cancer.