The NMR structure of the SRV-1 pseudoknot (26) revealed a number of interesting features that may be relevant to the process of frameshifting. These features include stacking of adenosines A21-A26 in loop L2 and flipping out of cytosine 24, base-base and/or base-sugar interactions of A21, A26, U27 and A28 from L2 with the minor groove of stem S1 ( Figure 1A ). To test the importance of these interactions in frameshifting, mutations were introduced in these regions of the pseudoknot and their frameshifting capacity was measured in rabbit reticulocyte lysates and compared to the NMR wild-type pseudoknot ( Table 1) .