In general, a pseudoknot is more efficient in stimulating frameshifting than a hairpin of the same sequence (18, R.C.L. Olsthoorn unpublished data). This difference is likely related to a higher thermodynamic stability of the pseudoknot. Indeed, from thermodynamic analyses it appears that pseudoknots are more stable than their hairpin counterparts (19) (20) (21) . Recent studies employing mechanical 'pulling' of frameshifter pseudoknots have shown a correlation between the mechanical strength of a pseudoknot and its frameshifting capacity (13, 14) and influence of major groove and minor groove triplex structures (22) . The higher strength of a pseudoknot can be primarily attributed to the formation of base triples between the lower stem S1 and loop 2 (Figure 1 ), making it more resistant against unwinding by an elongating ribosome (15, 23) . Base triples in the pseudoknots of Beet western yellows virus (BWYV) (24) , Pea enation mosaic virus (PEMV-1) (21) and Sugarcane yellow leaf virus (ScYLV) (25) have been shown to play an essential role in frameshifting.