Over the past 20 years, with the emergence of HER2-targeted agents, it has been proved that the addition of 1-year trastuzumab to adjuvant chemotherapy significantly reduces the recurrence rate and prolongs survival in patients with HER2-positive breast cancer (Perez et al., 2014; Cameron et al., 2017; Wolff et al., 2013). This evidence concerns the gene ERBB2 and breast cancer.