Human epidermal growth factor 2 (HER2) is overexpressed in approximately 15% of all breast cancer (BC) cases in the United States.1 Median overall survival (OS) of HER2-positive (HER2+) metastatic breast cancer (MBC) was less than 2 years prior to the advent of targeted therapies.2 These rapidly evolving treatments used alone or in combination, along with the discovery of novel biomarkers has translated into enormous progress with regard to outcomes,3 and description of long-term outcomes is feasible. This evidence concerns the gene ERBB2 and breast carcinoma.