More importantly, a positive CK5/6 staining in ≥ 10% of the HER2+ breast cancer tissue sections remained the sole independent factor predicting a worse outcome of trastuzumab-treated patients in terms of TTF in multivariate Cox regression analysis (hazard ratio 6.80, 95% confidence interval 1.39–33.36). The gene discussed is ERBB2; the disease is breast cancer.