RNase P, a tRNA processing enzyme, contains both RNA and protein subunits. M1 RNA, the catalytic RNA subunit of RNase P from Escherichia coli, recognizes its target RNA substrate mainly on the basis of its structure and cleaves a double stranded RNA helix at the 5' end that resembles the acceptor stem and T-stem structure of its natural tRNA substrate. Accordingly, a guide sequence (GS) can be covalently attached to the M1 RNA to generate a sequence specific ribozyme, M1GS RNA. M1GS ribozyme can target any mRNA sequence of choice that is complementary to its guide sequence. Recent studies have shown that M1GS ribozymes efficiently cleave the mRNAs of herpes simplex virus 1 and human cytomegalovirus, and the BCR-ABL oncogenic mRNA in vitro and effectively reduce the expression of these mRNAs in cultured cells. Moreover, an in vitro selection scheme has been developed to select for M1 GS ribozyme variants with more efficient catalytic activity in cleaving mRNAs. When expressed in cultured cells, these selected ribozymes also show an enhance ability to inhibit viral gene expression and growth. These recent results demonstrate the feasibility of developing the M1GS ribozyme-based technology as a promising gene targeting approach for basic research and clinical therapeutic application.