HER2 is a tyrosine kinase – human epidermal growth factor receptor. HER2 gene amplification and protein overexpression are found in 25-30% of patients with invasive breast cancer and 80% of patients with intraductal breast cancer. These breast cancers are highly invasive, have short survival, poor prognosis, and are insensitive to hormonal therapy and conventional chemotherapy. Trastuzumab (Herceptin) is a recombinant DNA-derived humanized monoclonal antibody that selectively acts outside HER2 cells and has better efficacy in HER2 overexpressed breast cancers. Since only breast cancer patients with HER2 gene amplification and protein overexpression are effective with trastuzumab, testing the HER2 status of breast cancer is crucial. HER2 protein and gene status should be tested in all patients with a clear pathological diagnosis of breast cancer. Patients with recurrence and metastasis should be tested again for recurrent and metastatic cancer. Currently, immunohistochemistry is generally used to detect HER2 receptor protein overexpression, while fluorescence in situ hybridization (FISH) is the main method to detect HER2 gene amplification level, which is currently the gold standard for detecting HER2 gene amplification level. Usually, breast cancer specimens are first tested for HER2 receptor protein overexpression by immunohistochemistry, and trastuzumab treatment can be applied directly for 3+. For patients with HER2 receptor protein overexpression <3+, further HER2 gene amplification detection by FISH is required, which not only provides a basis for the selection of trastuzumab therapy, but also helps to judge the prognosis of patients, which has important clinical significance.