Abstract: ObjectiveTo investigate the role of COX-2 in invasiveness and chemotaxis of the highly malignant breast cancer cell line MDA-MB-231 using RNA interference. MethodsMDA-MB-231 cells were transfected with small RNA interference plasmids to disrupt COX-2 expression. COX-2 mRNA level was detected using RT-PCR. The in vitro invasion ability of MDA-MB-231 cells was examined using matrigel invasion assay. ResultsThe results of restriction endonuclease digestion electrophoresis and DNA sequencing showed that the interferenced plasmid pSUPER-siCOX-2 was constructed successfully. The transfectants were named as MDA-MB-231/pSUPER-basic and MDA -MB-231/ pSUPER-siCOX-2, respectively. The results of RT-PCR showed that the levels of COX-2 mRNA of MDA-MB-231/pSUPER-siCOX-2 was obviously reduced at 48 hours after transfection, compared to MDA-MB-231/pSUPER-basic cells. The COX-2-reduced MDA-MB-231 cells showed decreased chemotaxis ability compared with the control cells (P<0.01). The invasion assay showed prominent differences between the COX-2-reduced MDA-MB-231 cells and the control cells (P< 0.01). Conclusion Using RNA interference， the reduction of COX-2 expression can obviously inhibit the invasion and chemotaxis of the highly malignant breast cancer cell line MDA-MB-231.