<正>Objective: To compare the cleavage activity of ribozymes directed against 2 sites of PDGF receptorP subunit cDNA gene in vitro. Methods: The 608 bp fragment of PDGF receptor β subunit cDNA was clonedinto T-vector under the control of T7 promoter

named pPDGFR-β. Two ribozymes were designed to cleavethe CUU sequence at codon 45 and codon 252 of PDGF receptor β subunit mRNA respectively. These 2 ham-merhead ribozyme genes were cloned into vector PI. 5 between 5’ -cis ribozyme and 3’ -cis ribozyme to gener-ate the plasmids of pRZ1 and pRZ2. The pPDGFR-β

pRZl and pRZ2 were linearized and then transcribedwith T7 promoter in vitro. Results: The RZ1 showed high cleavage activity in vitro

but the RZ2 showed nocleavage activity under the same condition. Conclusion: The cleavage site selection is an important factor in-fluencing the cleavage activities of ribozymes.