Further multivariable analyses, including age, HR status, Ki67 score and HER2 status and initial metastasis sites, showed that HER2-zero breast cancer patients had a worse OS than HER2-low breast cancer patients (HR: 1.48, 95% CI 1.12–1.95, P = 0.005) (Table 3). This evidence concerns the gene ERBB2 and breast cancer.