To investigate the expression and significance of proteasomes reactivator REG gamma (γ) in breast cancer. First, we showed the expression of REGγ in breast cancer, metastatic lymph nodes and normal breast tissues. Meanwhile, we also analyzed the relationship between REGγ and estrogen receptor (ER), CerBb-2, lymph nodes metastasis and clinical stage of breast cancer. REGγ expression was determined by immunohistochemical staining and western blot. Secondly, we detected the expression of REGγ and REGγ-mRNA in human breast cancer cell lines (MDA-MB-231, MCF-7) and human breast ductal epithelial cell line (HBL-100) by western blot and real-time PCR. Finally, in order to identify effect of REGγ on breast cancer cell cycle and proliferation, we constructed recombinant plasmid of PcDNA3.1-REGγ and designed siRNA for REGγ in vitro. Cell cycle was assayed by flow cytometer (FCM), proliferation was measured by methyl thiazolyl tetrazolium (MTT). The results demonstrated abnormal high expression of REGγ in breast cancer and its metastatic lymph nodes. REGγ expression was related to breast cancer and its status of ER, CerBb-2 and lymph nodes metastasis. REGγ is one of the potential markers in breast cancer. REGγ could facilitate the growth of breast cancer cells.