HER2 (Human Epidermal Growth Factor Receptor 2) is a protein that in humans is encoded by the ERBB2 gene.Amplification or over-expression of this gene has been shown to play an important role in the pathogenesis and progression of certain aggressive types of breast cancer and in recent years it has evolved to become an important biomarker and target of therapy for the disease. Amplification or over-expression of the ERBB2 gene occurs in approximately 30% of breast cancers. It is strongly associated with increased disease recurrence and a worse prognosis. Over-expression is also known to occur in ovarian, stomach, and aggressive forms of uterine cancer. HER2 proteins have been shown to form clusters in cell membranes that may play a role in tumorigenesis. HER2 is the target of the monoclonal antibody Herceptin which is effective only in cancers where HER2 is over-expressed. HER2 testing is performed in breast cancer patients to assess prognosis and to determine suitability for Herceptin therapy. Results so far have suggested that changes in serum HER2 concentrations may be useful in predicting response to Herceptin therapy.

Breast cancer threatens women all over the world, and has become the second cause for female deaths. In China, the incidence of breast cancer has been increasing and has become the most common female malignant tumor. HER2 is a member of the erbB-like oncogene family, and is related to, but distinct from, the epidermal growth factor receptor.This oncogene has been shown to be amplified in human breast cancer cell lines. Many studies show a close correlation between the expression of HER2/neu and prognosis in breast cancer. HER2 expression can serve as the reference index to select and estimate the adjuvant chemotherapy regimens. HER2 over-expression may suggest that patients will be resistant to CMF regimen, but sensitive to anthracyclines-based or anthracyclines plus taxanes regimen. Compared to CMF regimen, anthracyclines-based and anthracyclines plus taxanes regimens are more effective in HER2 over-expressed patients, especially the node-positive ones. However, HER2 over-expressed patients, even though node-positive, will probably respond to anthracyclines-based and anthracyclines plus taxanes regimens with similar sensitivity.