C20. For NMR purposes the wild-type G-residue at this position was replaced by a C (26) . Back mutation to a G (SF105) resulted in a 1.2-fold increase in frameshifting ( Figure 1C ). An adenosine at this position (SF106) was even more beneficial for frameshifting showing a 1.5-fold increase relative to the control NMR pseudoknot ( Figure 1C , compare lanes 'SF103 0 and '106 0 ). A uracil at this position was not tested since this would lead to a premature stopcodon in the À1 frame. Insertion of UUU (SF114) between C19 and C20 led to a slight decrease in frameshifting: 88% relative to the NMR wild-type (SF103). Even though in the latter mutant two additional base pairs might be formed with the 5 0 single-stranded sequence (AU and GU) these are probably melted when the ribosome is stalled over the slippery sequence.