Because the mere presence of a pseudoknot structure with a certain minimal thermodynamic stability might not be the sole primary determinant of frameshifting a number of alternative models have been proposed in which mechanical resistance of the pseudoknot to forced unwinding is the key (12, 15, 23) . In these models a pseudoknot trapped in the mRNA entry tunnel resists mechanical unwinding, which causes the ribosome to pause. Indeed, recent optical tweezers experiments, in which rupture forces of RNAs are measured by pulling 5 0 and 3 0 ends apart, showed a correlation between frameshifting and mechanical strength of the pseudoknot and confirmed a positive effect of base triple interactions on frame shift efficiency (13, 14, 22) . Frameshifting by the trapped pseudoknot is thought to be either passive, by stalling the ribosome for a sufficient amount of time, or active, by building up tension at the mRNA-tRNA interface by a counteracting ribosome, which can be relieved by a À1 frameshift (15) . Observations that ribosomes can stall for seconds to minutes without frameshifting to occur (35) and pausing appears to be necessary, but not sufficient for frameshifting (36, 37) , rather point to an active role. Although it now seems mechanical resistance to RNA unfolding is the key to ribosomal frameshifting, how the ribosome exactly 'chokes' on such a structure is still far from being understood. Remaining questions are for instance if torsional restraint of the pseudoknot resisting ribosomal unwinding is essential (23) , whether the pseudoknot truly plays an active role in for instance overbending the tRNA in the P-site (12) and at which stage of translational elongation frameshifting actually occurs.