Objective   To investigate the inhibitory effect of CXC chemokine receptor 4 (CXCR4) siRNA on the growth of transplanted tumors of esophageal squamous carcinoma in nude mice.   Methods   Nude mice model bearing esophageal squamous carcinoma was reproduced by implanting human esophageal squamous carcinoma cell line EC-9706. CXCR4 siRNA was injected to the area nearby the tumors. The tumors injected with control siRNA or PBS served as control. Tumor growth and tumor weight were measured. Histopathological changes of the tumors were observed with HE staining

and the mRNA and protein expressions of CXCR4 were detected by RT-PCR and immunohistochemical staining respectively.   Results   Tumor growth was obviously suppressed and tumor volume was significantly smaller in the group treated by with CXCR4 siRNA than in control groups. While the histopathological features in the control group showed that the tumor volumes were bigger and large amount of atypic tumor cells was seen. RT-PCR and immunohistochemical staining indicted that CXCR4 mRNA and protein expressions were reduced in in the group treated by with CXCR4 siRNA.   Conclusion   CXCR4 siRNA inhibits the tumorigenesis of EC9706 cell xenograft in nude mice and down-regulates the CXCR4 mRNA and protein expressions in vivo.