The invention found that overexpression and activation of α9-nAChR are associated with tumorigenesis of breast cancer and create a number of small interfering RNAs to inhibit the expression of α9-nAChR so as to inhibit breast cancer. Therefore, the invention provides small interfering RNAs (siRNAs) for inhibiting expression of α9-nAChR so as to inhibit breast cancer, methods to prevent/inhibit/treat malignant progression of nicotine-derived-compound-induced breast cancer and method of determining malignant level of such breast cancer.
