G3-C17. This pair is also not involved in any triple interaction, but replacing it with A-U (SF342) resulted in a 60% drop in frameshifting. Again this indicates that the stability of S1 plays an important role in the frameshifting capacity of the pseudoknot. Previously, changing G3-C17 in the wild-type pseudoknot to A-U resulted in an almost 5-fold drop in frameshifting (31) . Since we do not have high-resolution structural information of the wild-type pseudoknot (L2: GAAACAAGCUUA), we cannot exclude that the G3-C17 base pair forms a base triple in this context, which could further attribute to the enhanced effect of mutating this base pair.