Upon stratification of basal CK5/6 expression pattern, univariate analysis revealed 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 (hazard ratio 5.45, 95% confidence interval 2.07–14.35; P < 0.001). This evidence concerns the gene ERBB2 and breast carcinoma.