The result of quantifying the fraction of in vivo À1 frameshifted ribosomes, both those which made it all the way to the lacZ stop codon (gene10/lacZ fusion) and those which stalled inside the pseudoknot, is shown in Figure 3A . The hatched bars denote the À1 frameshift efficiency taking into account only the end product of À1 frameshift (148 kDa gene10/lacZ fusion). This frameshift efficiency was calculated as (intensity of FS product)/ (intensity of non-FS product+intensity of FS product). The filled bars denote the À1 frameshift efficiency when both the end product (148 kDa gene10/lacZ fusion) and the products originating from stalled ribosomes are taken into account. This frameshift efficiency was calculated as (intensity of FS product+intensity of stalled product)/(intensity of non-FS product+intensity of FS product+intensity of stalled product).