If refolding kinetics of the pseudoknot is a determinant in frameshift efficiency the length of L2 is also expected to have an effect on frameshifting. To test this possibility we changed the length of L2 to 3 and 27 nt in mutants SF348 and SF350, respectively. The absolute frameshifting activity of SF348 was only 1.3%, i.e. a 16-fold decrease compared to SF206. SF350 showed 8.4% frameshifting, a 2.5-fold drop. Of course, when introducing such dramatic changes, one should also consider their effects on the pseudoknot structure and stability. In SF348 most of the base triples are presumably lost and a 3-nt loop crossing the minor groove of stem S1 may not be sterically and thermodynamically favorable. In fact, the very large-16-fold-reduction in frameshifting suggests that the pseudoknot is not formed at all (the presence of a pseudoknot structure was not checked by e.g. structure probing).