Currently, the 2018 American Society of Clinical Oncology (ASCO) [8], the 2017 US National Comprehensive Cancer Network (NCCN) Guideline on Breast Cancer [9], and the 2015 European Society for Medical Oncology (ESMO) Clinical Practice Guidelines for the Diagnosis, Treatment, and Follow-Up of Primary Breast Cancer [10] recommended the administration of trastuzumab (the first monoclonal antibody-based therapy developed to specifically target HER2) with chemotherapy for the management of HER2-positive early-stage breast cancer (EBC) in adjuvant settings. This evidence concerns the gene ERBB2 and breast cancer.