ERBB2 and breast cancer: Approximately 15% to 20% of breast cancer patients exhibit overexpression of human epidermal growth factor receptor 2 (HER2) protein and/or amplification of its gene.1 The randomized phase III CLinical Evaluation Of Pertuzumab And TRAstuzumab (CLEOPATRA) trial evaluated the efficacy of adding pertuzumab to trastuzumab and docetaxel as a new first-line standard.2 It reported significant improvements in both progression-free survival progression-free survival (PFS) and overall survival (OS) with dual HER2 blockade.