ERBB2 and breast cancer: Remarkably, after accounting for prognostic variables correlating with univariate survival at p < 0.20 (Table 5), tumor size and the presence of the basal 2-HER2+ phenotype (i.e., a positive CK5/6 staining in ≥ 10% of the HER2+ breast cancer tissue sections) retained their independent value for predicting a worse prognosis in terms of TTF (hazard ratio 3.66; 95% confidence interval 1.24–10.78; P = 0.019) in multivariate Cox regression analysis.