We have designed and synthesized a hammer-head Rz targeted to human proliferating cell nuclear antigen (PCNA) mRNA at site No.524. Results showed that Rz524 can cleave the transcribed substrate RNA site-specifically in vitro. A self trimming expression plasmid of this Rz was constructed and co-introduced into HeLa cells with T7 vaccinia virus. Dot-blotting hybridization showed that the Rz was expressed in HeLa cells.