ERBB2 and breast cancer: 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%) predicted a significantly worse prognosis in terms of 5-year OS (hazard ratio 4.07, 95% confidence interval 1.88–8.79; P < 0.001).