To quantify the amount of ribosomes stalling within a pseudoknot in vivo we calculated the ratio of (stalled+non-stalled frameshifted ribosomes) to (non-stalled frameshifted ribosomes), the result is shown in Figure 3B . For the IBV inspired pseudoknots, this ratio was close to 1 signifying that essentially no ribosomes stalled. However, the ratio was significantly larger than 1 for the more artificial pseudoknots which acted as roadblocks for a large amount of frameshifted ribosomes. The length of stem 1 did not significantly influence on the amount of frameshifted or stalled ribosomes. Interestingly, within pseudoknots with the same overall structure (22/6a-c) 22/6a stalls a significantly higher fraction of frameshifted ribosomes than 22/6b (verified by Student's t-test, n = 4, a = 0.05, P = 0.012), which again stalls more than 22/6c. Hence, the ability to stall a ribosome correlated with the strength of the pseudoknot base pairs, the stronger the base pairs the more frameshifted ribosomes were stalled.