Our results indicated that BMP9 inhibited the tumor growth of xenografts in vivo, down-regulated leptin expression, inhibited the phosphorylation of STAT3, ERK1/2, AKT, and reduced the expression of c-Myc, CyclinD1 and MMP9 in the breast cancer, while opposite results were observed in Co-pre-siBMP9 group and Co-adi-siBMP9 group. The gene discussed is MMP9; the disease is neoplasm.