Several naturally occurring ribozymes have now been well characterized with respect to their in vivo and in vitro activities. Through detailed biochemical and genetic analyses, it has become possible to alter the substrate specificity of each ribozyme using simple Watson-Crick base pairing. Several laboratories, therefore, have designed ribozymes to cleave viral or other cellular transcripts in vitro with the hope of developing these molecules as antiviral or therapeutic agents. In addition to Watson-Crick base pairing, however, other factors such as protein or RNA tertiary interactions are involved in the ribozyme cleavage activity. Although several engineered ribozymes have been used successfully to reduce gene expression in vivo, it is difficult to determine whether gene expression has been reduced by the cleaving activity of the ribozyme or by its inherent antisense activity. In order to discriminate between these two activities and optimize potentially therapeutic ribozymes, it is imperative to develop in vivo assays in which the antisense activity of ribozymes is negligible.