Inhibition of gene expression is an important experimental method to determine the function of genes, and could have a great impact upon the treatment of diseases in which gene products are involved. Catalytic RNAs (ribozymes) with endoribonuclease activity are RNA molecules that site-specifically cleave other RNAs, and thus have tremendous potential as novel therapeutic agents. The capacity of these agents to suppress gene expression in a wide range of systems strengthens the importance of this novel gene-based therapy, and indicate that preformed ribozymes may be useful as pharmaceuticals.