Abstract: Objective: To investigate the factors which affect the expression of BRCA1 protein in sporadic breast cancer and to analyze the relationship between these factors and clinicopathological parameters. Methods: The expression level of BRCA1 protein, ER α and HER-2 was detected by immunohistochemistry (IHC) with Streptavidin-Peroxidase (SP) method in benign breast tissues and sporadic breast carcinoma tissues. Clinical data from patients were analyzed by Spearman correlation test and corrected Pearson χ² test. Results: In the benign breast tissues, the positive expression rates of BRCA1 and ER α protein in nuclei were 100%(20/20) and 80%(16/20), respectively. HER-2 expression was 15.0%(3/20) in the cytoplasm. In the sporadic breast cancer tissues, the expression rates of BRCA1 and ER α protein in nuclei were 57.5%(88/153) and 64.1% (98/153), repectively. HER-2 expression was 34.0%(52/153) in the cytoplasm. The difference in BRCA1 expression between the two groups was significant (P<0.05). No significant difference was found in ER α and HER-2 expression between the two groups. Loss of BRCA1 protein expression was associated with loss of ERαexpression and HER-2 overexpression. Loss of ER α expression was correlated with HER-2 overexpression. High rate of loss of BRCA1 expression was found in the breast cancer tissues with loss of ER α expression and HER-2 overexpression. Furthermore, loss of BRCA1 expression in breast cancer tissues was correlated with the age of the patients and metastasis to axillary lymph nodes. There was no distinct association between loss of BRCA1 expression and pathological types of breast cancer. Conclusion: In sporadic breast cancer, loss of BRCA1 expression in the nuclear is correlated with loss of ER α expression, HER-2 overexpression, age, and metastasis to axillary lymph nodes and it may involve in the occurrence and development of breast cancer.