To investigate whether the mutations that affected frameshifting efficiency in vitro would also exert their effect in vivo, we introduced some of the above mutations into a dual luciferase reporter plasmid and assayed their activity in vivo ('Materials and Methods' section). The frameshifting efficiency of the wild-type SRV-1 pseudoknot and GGGAAAC slippery sequence (SF400) was only $8%, against a background of 2% of a non-frameshifting control (data not shown). This is a factor of 3 lower than that reported in vitro (28) . The NMR pseudoknot showed $11% of frameshifting (SF402, data not shown) that increased to $19% in combination with the UUUAAAC slippery sequence (SF404). This 1.7-fold increase is very similar to the previously reported increase from 23 to 40% in vitro for changing the slippery sequence (28) . Mutations were made in the pseudoknot of plasmid SF404. As can be seen in Figure 3 , the substitutions had almost the same effect on frameshifting in vivo as in vitro. The only exception seems to be the effect of enlarging L2, which is consistently more detrimental for frameshifting in vivo than in vitro. This could be due to refolding kinetics of the pseudoknot, what in combination with heavier ribosomal traffic in vivo would result in a lower fraction of properly folded pseudoknots. However, other possibilities, like enhanced RNase susceptibility or alternative folds involving L2, may also account for this effect.