With the advent of antisense technology, there has been much interest in the use of long RNAs expressed in vivo to inhibit the expression of target genes. More recently, there have been numerous reports that the incorporation of either hairpin or hammerhead ribozyme motifs (catalytic antisense) into such RNAs increases their effectiveness (1 –3 ). This section will describe a generally applicable, simple, PCR-based method to construct catalytic antisense RNA containing the hammerhead catalytic core that will cleave the target of interest at a GUC sequence. All discussion will focus on the hammerhead motif, although, in principle, the hairpin motif could be incorporated instead.