ERBB2 and breast cancer: Based on the high heterogeneity of HER2 mRNA expression within HER2-positive tumors [16,19,20,21,22], we first validated HER2 expression by qRT-PCR in our cohort of HER2-positive BC patients, treated in adjuvant setting with trastuzumab, for which we have gene expression data that we used to develop the TRAR score ([10], GHEA, n = 36).