The utilization of T-DM1 in patients with HER2+ breast cancer has recently increased, as clinical trials have demonstrated T-DM1 to be a well-tolerated and effective treatment for patients with HER2+ breast cancer with residual disease after neoadjuvant treatment [17] and in patients with HER2+ advanced or metastatic breast cancer [7, 18]. This evidence concerns the gene ERBB2 and breast cancer.