Treatment with trastuzumab (Herceptin) or other human epidermal growth factor receptor 2 (HER2)-targeted treatment regimens reduces the risk of cancer recurrence and all-cause death in patients with HER2-positive breast cancer but is associated with cardiotoxicity, which manifests as clinical heart failure or left ventricular ejection fraction (LVEF) decline [1–3]. This evidence concerns the gene ERBB2 and breast cancer.