Previously, we have demonstrated that antisense oligonucleotides can induce high levels of À1 frameshifting (27) . The optimal length of small antisense oligonucleotides, however, was not investigated. Understanding the optimal length of trans-acting oligonucleotides that can induce the most efficient frameshifting and, at the same time, escape RNAi interference will be an important issue for future in vivo applications. Here we found that maximum levels of frameshifting were obtained with oligonucleotides of 12 nt and more. This is comparable to the stem lengths (S1+S2) of known examples of highly frameshift inducing H-type pseudoknots, such as the 6+6 bp of the Simian retrovirus typeÀ1 pseudoknot (29) , the 11+6 bp of the minimal Infectious Bronchitis virus (IBV) pseudoknot (30) , and the 6+6 bp chimeric Mouse Mammary Tumor virus (MMTV)-IBV pseudoknot (31) . In addition, in known examples of hairpin-induced frameshifts, the stem length of hairpins is around 12 bp (32) (33) (34) . This may imply that a full helical turn of an RNA helix either in one single stem or in two stacking stems of a pseudoknot (S1+S2) is selected by viruses to induce efficient ribosomal frameshifting.