The two Cox proportional-hazards models given in Table 4 suggested that HER2 ECD level was a robust independent prognostic factor in patients with tissue HER2-positive breast cancer, irrespective of the variable tentative cutoff values: elevated vs normal (HR = 3.788, 95% CI = 2.034–7.058, P < 0.001, Table 4) and third/highest quartile vs lowest quartile (third quartile: HR = 3.029, 95% CI = 1.003–9.152, P = 0.049; highest quartile: HR = 4.746, 95% CI = 1.591–14.160, P = 0.005; Table 4). This evidence concerns the gene ERBB2 and breast cancer.