The structures analysed in this study are artificial and were designed to fold into pseudoknot-like structures with a gradually increasing mechanical strength. The mechanical strength was adjusted by changing the base pairs of the two stems, which seems to be a reasonable way of crudely varying the mechanical strength, as the energy involved in base pairing is higher than the energies involved in, e.g. the electrostatic interaction of the loop with the stems. It is, however, likely that the loop-stem interaction, surface charges or other players than just mechanical strength influence frameshift stimulating effect of mRNA structures. As there is a consensus in recent literature that pseudoknot mechanical strength correlates with frameshifting efficiency (23) (24) (25) , it was intriguing that the amount of frameshifted product was reduced by the stronger pseudoknot 22/6a compared to the weaker 22/ 6b or c ( Figure 3A ). This proved to be caused by stalling of a significant amount of frameshifted ribosomes by the strong pseudoknots ( Figure 3B ). Future studies will show whether significant stalling can also be caused by naturally occurring pseudoknots.