Therefore, our results suggested that serum HER2 could be a clinical biomarker for HER2 subtype conversion from the primary HER2-negative tumor during metastasis or disease progression, which provided clinical evidence that HER2-targeted therapy could be useful for patients with recurrent HER2-negative breast cancer who were diagnosed with a high level of serum HER2. This evidence concerns the gene ERBB2 and breast carcinoma.