Human epidermal growth factor receptor 2 (HER2)‐positive breast cancer (BC) is defined by high expression levels of the HER2 tyrosine kinase receptor as determined by immunohistochemistry (IHC) and/or amplification of the HER2 gene as evidenced by fluorescence in situ hybridization (FISH; Loibl & Gianni, 2017; Ménard et al., 2000). This evidence concerns the gene ERBB2 and breast cancer.