ERBB2 and breast cancer: Moreover, multivariable Cox regression analysis including all clinical variables significant in any univariable analysis demonstrated ERBB2 CN to be an independent biomarker for poor outcome in trastuzumab-treated primary breast cancer with a hazard ratio (HR) of 3.3 (95% CI 1.1–9.6; p = 0.031) for RFS and HR 3.6 (95% CI 1.1–12.6; p = 0.041) for OS (Fig. 4b, d).