The main role of the triple interactions seems to be in stabilizing the pseudoknot such that high frequencies of frameshifting are possible. Mutations that remove triple interactions in general lead to lower frameshifting. Conversely, it seems that frameshifting pseudoknots with weaker stems need to be stabilized by such triples. For instance in the BWYV and PEMV-1 pseudoknots, S2 is only three base pairs but this is compensated by an additional triple between L1 and S2. The high number of triples in luteovirus pseudoknots may explain why for instance the ScLYV pseudoknot with a relatively high number of A-U base pairs in the bottom of S1 is still capable of inducing $15% of frameshifting (25) . On the other hand, when S1 is increased to 11 base pairs, triple interactions do not seem to contribute anymore as frameshifting becomes independent of the sequence of L2 (C-H. Yu and R.C.L Olsthoorn, unpublished data).