We created a series of plasmids containing different pseudoknots and where the in-frame stop codon was placed either immediately upstream ('Upstream stop') or $150 nt downstream ('Downstream stop') from the pseudoknot ( Figure 1A ). The 'Upstream stop' constructs had an in-frame stop codon in the spacer between the slippery sequence and the pseudoknot. This caused non-frameshifted ribosomes to produce a 28 kDa polypeptide (gene10 from phage T7) while ribosomes undergoing a À1 frameshift continued through the pseudoknot and into lacZ producing a 148 kDa fusion protein of the T7 gene10 and lacZ sequences. In the 'Downstream Stop' constructs we replaced the UAA stop codon immediately upstream from the pseudoknot with a lysine codon (AAA). This change caused non-frameshifting ribosomes to continue through the pseudoknot and terminate at a downstream UGA codon producing a 37 kDa polypeptide. The pseudoknot constructs based on the plasmid OFX302 (25) are detailed in Figure 1B . We systematically increased the length of stem 1 and in pseudoknot 22/6a through 22/6c, we exchanged GC with UA base pairs, thus, gradually decreasing the stability of stem 1.