Abstract: Objective:To detect the expression of human endogenous retrovirus K (HERV-K) envelope (env) protein in breast tumors; to analyze the association between the expression level of HERV-K env protein and clinicopathologic char-acteristics of breast cancer; to explore the relationship between HERV-K env protein expression and the prognosis for pa-tients with breast cancer. Methods:Immunohistochemistry was used to detect the expression of HERV-K envelope protein in 95cases of breast cancer and 30cases of benign breast tumor tissue. The χ 2 test was used to analyze the relationship between the level of expression and clinicopathological characteristics.Results: Immunohistochemical staining results re-vealed that the positive expression of HERV-K env protein was evident in the cytoplasm. Breast cancer tissue showed me-dium or high expression in almost all cells. However, in benign breast tumor tissue the expression level was low or negligi-ble. The positive expression rate of HERV-K env protein was 88.4% (84/95) in breast cancer and 40.0% (12/30) in benign breast tumor tissues. The expression of HERV-K env protein in breast cancer was significantly higher than that in benign breast tumor tissue (P<0.001 ). The expression level of HERV-K env protein was correlated with some prognostic factors. Among the patients with breast cancer who had positive expression of HERV-K env protein, the expression level had a posi-tive correlation with tumor size, clinical stage and the number of positive axillary nodes, whereas, it had a negative correla -tion with the protein expression of ER ( P<0.05). No correlation was found between its expression level and age, histological grade, or the protein expression of PR in the patients with breast cancer (P>0.05). Conclusion:The expression of HERV-K env protein was negative in most benign breast tumors, but positive in almost all malignant breast tumors. Thus it may be correlated with the development and prognosis of breast cancer. The expression level of HERV-K env protein might serve as a prognostic marker for breast cancer.