Upon stratification of basal CK5/6 expression pattern, univariate analysis confirmed that a positive CK5/6 staining in ≥ 10% of the HER2+ breast cancer tissue sections (but not a positive CK5/6 < 10%) was a significant predictor of worse outcome in terms of TTF in trastuzumab-treated patients (hazard ratio 7.49, 95% confidence interval 1.41–39.70; P = 0.018). This evidence concerns the gene ERBB2 and breast cancer.