Abstract: Objective: To study the effect of specific inhibition of Semaphorin 4C (Sema4C) gene expression by small interfering RNA (siRNA) on the migration, invasion and proliferation of highly metastatic human breast cancer cell line MDA-MB-231 , in vitro. Methods:The Sema 4C siRNA was designed by web based software, synthesized and then transfect -ed into MDA-MB- 231 cell line by lipofectamine 2000 reagent. Western blot was used to analyze the effect of the siRNA transfection. The migration, invasion and proliferation of the Sema 4C-siRNA transfected cells were assessed by monolayer wound healing assay, transwell cell invasion assay and CFSE-flow cytometry. The expression of p-AKT protein in untrans-fected cells, negative control-transfected MDA-MB- 231 cells and Sema 4C-siRNA-transfected MDA-MB- 231 cells were stud -ied by Western blot analysis. Results: Significant down-regulation of the Sema 4C protein was found at72hours after trans-fection of the Sema 4C-siRNA. Compared with those of untransfected cells and negative control transfected cells, the migra-tion, invasion and proliferation ability of Sema 4C-siRNA-transfected MDA-MB- 231 cells were notably weakened. The pro-tein level of p-AKT was lower in the Sema 4C-siRNA-transfected MDA-MB- 231 cells. Conclusion:The in vitro transient trans-fection of synthesized Sema 4C-siRNA on the MDA-MB- 231 cell line can down-regulate the expression of Sema4C protein in the cells. Sema 4C-siRNA plays a vital role in suppressing migration, invasion and proliferation of MDA-MB-231 cells, which may be related to PI 3K-AKT cell signal pathway.