OBJECTIVE To construct a CXCR4 specific recombinant plasmid vector and study its inhibiting effect on invasion capacity in vitro of human breast cancer MDA-MB-231 cell line and its metastatic potential to the lung in nude mice. METHODS A CXCR4 specific recombinant plasmid vector was constructed and transfected into the cultured MDA-MB-231 cell line with lipofectamine 2000. RT-PCR and Western blot were used to detect the mRNA and protein expression of CXCR4, respectively. Invasion capability in vitro of the cells was evaluated by Boyden chamber. The cell proliferation capacity was detected by MTT method. The nude mouse model of lung metastasis was established by injection of MDA-MB-231 cells into the tail vein. The animals were sacrificed at 6 weeks after the tumor cells injection. Whole lung tissues were harvested, embedded in paraffin, sectioned serially, and the HE-stained paraffin sections were examined pathologically to evaluate the presence and number of metastatic tumors. RESULTS The CXCR4 mRNA expression rate was 29.5% +/- 3.8% in the CXCR4-shRNA group, significantly lower than that of the control group (69.7% +/- 2.6%, P < 0.01) and mock-control group (67.8% +/- 3.5%, P < 0.01). The CXCR4 protein expression rate was 15.4% +/- 1.1% in the CXCR4-shRNA group, significantly lower than that of the control group (39.0% +/- 2.4%, P < 0.01) and mock-control group (35.9% +/- 3.9%, P < 0.01). Silencing of CXCR4 by shRNA lead to a significant decrease in breast cancer cell invasion and proliferation capacity in vitro. Furthermore, tumor cells with CXCR4 shRNA permanent transfcetion had a much lower lung metastatic potential in nude mice than control cells and mock control cells in vivo. CONCLUSION CXCR4 shRNA can inhibit the expression of CXCR4 and decrease the invasion and lung metastatic potential of human breast cancer cells.