Abstract: Objective: To study the effect of liposome targeting ribonuclease inhibitor(RI) on the proliferation and cell cycle of breast cancer cell line MDA-MB-231.Methods: By the induction of liposome,pLNCX-RI and eukarytic expression vectors pLNCX were transfected into breast cell line MDA-MB-231.Then the subclone cell lines MDA-MB-231-231/pLNCX-RI which high expressed RI stably were obtained by persistent G418 selection.Western-blot and RT-PCR were used to detect RI expression level in transfected cell line;Cell prolifetation was analyzed by MTT assay;Cell cycle was analyzed by flow cytometry.Results: The subclone cell line MDA-MB-231-231/pLNCX-RI which high expressed RI stably was successfully selected.MTT showed the cell proliferation was inhibited(P<0.01).Cells stopped at G₀/G₁ phase and cell number in S phase was sharply decreased by the induction of liposome(P<0.01).Conclusions: The malignant proliferation of breast cancer cells is inhibited by RI.