One should also keep in mind the possible formation of alternative structures that can affect the frameshifting efficiency by lowering the fraction of properly folded pseudoknots. This possibility has not been investigated in detail but our own data ( Figure 2 ) suggest that this may be a relevant issue. Especially during heavy ribosomal traffic, as may be the case in vivo, fast and correct refolding of pseudoknots could be an important parameter for frameshifting. Under such circumstances it is conceivable that a hairpin is more preferred as it probably refolds faster than a pseudoknot. A recent study on the HIV-1 frameshift signal has shown that a decrease in the translation initiation frequency can lead to an increase in the frequency of frameshifting (38) . Also a previous study in yeast has shown a correlation between frameshifting and translation initiation frequency (36) . Our data with the enlargement of L2 also hint at a possible role for folding kinetics.