RNA enzymes, or ribozymes, are fascinating molecules with novel biochemical properties. Since the discovery of ribozymes in 1981, biological catalysis by RNA has been found to be at the heart of critical steps in gene expression, including RNA splicing, endonucleolytic RNA processing, and possibly ribosomal peptide bond formation (Sheldon et al. 1990; Noller et al. 1992; Symons 1992; Cech 1993; Long and Uhlenbeck 1993). In addition, ribozymes with altered substrate specificities have considerable potential as experimental tools for modulating gene expression through the cleavage of targeted mRNA molecules, and this potential may some day extend to therapeutic applications (Cech 1988; Haseloff and Gerlach 1988; Rossi and Sarver 1992).