Objective：To explore the alterations of  Cdc42 in adriamycin(Adr)-sensitivity and adriamycin-resistance MCF-7 human breast cancer cell line and the effect of Cdc42 on cells drug resistance.

Methods：Cdc42 siRNA was transfected  into MCF-7/Adr cells. Then, RT-PCR and Western blot were employed to examine the expression of Cdc42 protein and mRNA in MCF-7, MCF-7/Adr and MCF-7/Adr treated with Cdc42 siRNA; and multiple drug-resistance was detected by MTT colorimetric assay. Fluorospectorphotometer was used to detect the concentration of ADM in cells. 
Results：The expression of Cdc42 protein and mRNA in MCF-7/Adr was significantly higher than in MCF-7(P＜0.05), and MCF-7/Adr treated with siRNA  inhibited the expression of Cdc42 protein and mRNA; the IC50 in siRNA group was lower than that in MCF-7/Adr group, and the concentration of ADM  in siRNA group was significantly increased.
Conclusions：Cdc42 siRNA can significantly inhibit the expression of Cdc42 protein and mRNA and reverse multidrug resistance (MDR).