Although antisense oligonucleotides were found to induce ribosomal frameshifting (27, 28) , the optimal number of base pairs has not been addressed yet. To investigate this we designed antisense RNA oligonucleotides that are 6, 9, 12, 15 and 18 bases complementary to the region downstream of an UUUAAAC slippery sequence in our reporter plasmid SF468 ( Figure 1 ). First, titration with RNA6 and RNA9 oligonucleotides revealed that a 625-fold molar excess of oligonucleotides over mRNA resulted in the highest level of frameshifting ( Figure 2a) ; this ratio was used in the following experiments. The shortest oligonucleotide, RNA6, was not capable of inducing significant levels of frameshifting (Figure 2b) , whereas RNA9 induced $3.5% of frameshifting. Maximum levels were obtained with RNA12, RNA15 and RNA18; all three induced $12% of frameshifting. In the following experiments oligonucleotides between 12 and 18 nt in length were used.