Several models attempting to explain pseudoknot stimulation of programmed À1 frameshifting have been proposed [for reviews see (18, 19) ]. Most models invoke a pausing mechanism whereby the ribosome is paused over the shift site such that time is allowed for the tRNAs to reposition in the new reading frame. This explanation is clearly too simplistic as stem-loops and pseudoknots of similar thermodynamic stability that cause ribosome pausing are not necessarily effective frameshift stimulators (47) (48) (49) . In addition, variations of the IBV pseudoknot have demonstrated a lack of correlation between the extent of pausing and the efficiency of frameshifting (47) . A recent publication by Brierley and co-workers (50) presents structural data demonstrating that the IBV frameshift stimulating pseudoknot blocks the mRNA entrance tunnel and leads to a structural deformation of the P-site tRNA. The resulting movement of the tRNA displaces the anticodon loop towards the 3 0 end of the mRNA. A model is presented in which this movement results in disruption of the codon-anticodon interactions, thus allowing for tRNA slippage relative to the mRNA. Similar tRNA movements were not observed with non-frameshift stimulating stem-loop structures. This model provides a feasible mechanistic explanation for the ability of some downstream structures to induce frameshifting.