Recent single molecule investigations suggest that the mechanical strength of pseudoknots correlate with the ability of the pseudoknot to stimulate frameshift (25) (26) (27) , at least in a certain interval. However, the calculated Gibbs free energy does not always correlate with frameshift efficiency. Not only the strength of the stems, but also the interaction between the loop and the stems might be of importance for the ability to induce frameshift and for the overall mechanical strength and brittleness of the structure. If the pseudoknot becomes too strong the ribosome, frameshifted or not, might not be able to open it and continue translation, whereby the pseudoknot acts as a roadblock. Often in literature (25) (26) (27) (28) (29) (30) (31) frameshifting assays were performed on constructs exhibiting the common feature that the stop codon for the normal reading frame was located at the entrance of the pseudoknot (or inside the pseudoknot) and the stop codon for the successful À1 frameshift was located downstream of the pseudoknot. In most frameshifting assays, the amount of frameshifting is determined by quantifying the amount of full-length frameshifted versus non-frameshifted products. However, for this to be a correct measure, the frameshifted ribosome must continue translation through the pseudoknot and beyond to the À1 frameshifted stop codon. If the À1 frameshifted ribosome permanently stalls inside the pseudoknot, it would falsely be interpreted as if the ribosome did not frameshift. Therefore, there is a serious pitfall in the classical methods which renders the amount of frameshifted ribosomes to be non-correctly determined, i.e. be underestimated, potentially leading to false hypotheses regarding the physical mechanism of frameshifting.