Objective：To study the reversal effecy of small interfering RNA (siRNA) on multidrug resistance and chemotherpy sensitivity of xenografts human breast cancer tumor, and  chemotherpy sensitivity in nude mice.
Methods：siRNA targeted on multidrug resistance protein(MRP) was transfected into MCF-7 cell xenografts tumors of mice, then the expression level of the MRP mRNA and protein was determined, and the chemotherapeutic sensitivity of the breast cancer tissues was evaluated.
Results：The RT-PCR and immunohistochemistry results showed that the level of MRP mRNA and protein decreased in the experimental group 24 h after the siRNA was transfected (P＜0.05). The tumor inhibitory rate of epirubicin was significantly increased in the siRNA transfection group compared to the control group(42.2% vs. 11.9%).
Conclusions：siRNA targets to MRP gene can inhibit the expression of MRP mRNA and protein in xenograft breast cancer in nude mice,  results in the txenograft tumor sensitivity to epirubicin is obviously increase, and the multidrug resistance of tumor is reversed partially.