ERBB2 and breast cancer: Notably, the HER2‐enriched BC subtype that includes HER2‐positive BC cases more susceptible to trastuzumab treatment (Prat et al., 2014; n = 16) was characterized by significantly higher HER2 mRNA levels in comparison with the other BC subtypes considered all together (n = 37; Figure 2; p = 2.5 × 10−7) or separately (Supporting Information Figure 2; p = 4.6 × 10−5), In addition, GHEA relapsed cases had significantly lower HER2 mRNA levels than those relapse‐free (p = 1.1 × 10−9; Figure 2b).