Abstract: Objective:To assess the expression of retinoic acid receptor beta 2 (RAR-β2) in breast tumor and to evaluate the relationship between RAR-β 2 expression and tumorigenesis of breast cancer.Methods: Immu -nohistochemistry was used to detect the expression of RAR-β 2 protein in specimens from 40cases of breast cancer,40cases of atypical ductal hyperplasia, 40cases of fibroadenoma and 20cases of normal breast tissues. Reverse transcription polymerase chain reaction (RT-PCR) was used to detect the mRNA expression levels of RAR-β 2 gene in 20cases of breast cancer, 20cases of atypical ductal hyperplasia,20cases of fibroadenoma, and 20cases of normal breast tissues. Results:Immunohistochemical staining results revealed that the positive expression of RAR-β 2 protein showed nuclear staining. The positive expression rate of RAR-β 2 was 30% (12/40) in breast cancer, 17.5% (7/40) in atypical ductal hyperplasia, 87.5% (35/40) in fibroadenoma, and 95% (19/20) in normal breast tissues. The expression of RAR-β 2 protein in breast cancer was significantly lower than that in normal breast tissues (χ2=26.30, P<0.001 ). The expression of RAR-β 2 was not significantly different between atypical ductal hyperplasia and breast cancer ( P>0.05）. No correlation was found between the expression of RAR-β2 protein and the tumor size, menopausal age, lymph node metastasis, clinical stage, histological grade, or protein expression of ER and PR in breast cancer tissues ( P>0.05). Follow-up results showed that 3 out of 28patients with negative RAR-β 2 expression had visceral organ metastasis, but only one of the 12RAR-β 2 positive patients had osseous metastasis. RT-PCR analysis showed that the positive expression rate of RAR-β 2 mRNA in breast cancer, atypical ductal hyperplasia, fibroadenoma and normal breast tissues was 25% (5/20), 35% (7/20), 85% (17/20) and 100 % (20/20), respectively. The RAR-β2 mRNA expression rate in breast cancer was significantly lower than that in normal breast tissues ( χ2=30.43, P<0.001 ). No significant difference in RAR-β2 mRNA expression was found between atypical ductal hyperplasia and breast cancer tissues ( P>0.05). Conclusion: RAR-β2 gene may play a repressive role in the initiation of breast cancer, and the loss of the expression of RAR-β2 gene may be the initial step in breast carcinogenesis.