Deletion of the bulging C24 from the loop L2 did not affect frameshifting significantly in vitro and in vivo. This adds to the notion that this nucleotide can be safely squeezed out to allow stacking of adenosines in L2. It is noteworthy that other frameshifter pseudoknots also have a cytosine flanked by adenines in L2: GAACAAA in Figure 3 . Comparison of in vivo and in vitro frameshifting activities relative to that of the NMR pseudoknot. White bars, in vivo; gray bars, in vitro. The efficiency of the NMR pseudoknot ('wild-type') is set at 100%. Experiments were done at least three times in triplicate.