Our results are in agreement with the observation that the amount of protein produced from an mRNA can be reduced when a pseudoknot is located upstream (29) . Also, they provide a possible explanation for the reduction in frameshift efficiency observed by, e.g. Napthine et al. (30) when increasing the thermodynamic stability of stem 1 above a certain threshold. This apparent reduction in frameshift efficiency (observed by 1D SDS-PAGE) could be caused by the fact that a significant fraction of the 'frameshifted' ribosomes permanently stalled within the pseudoknot.