Despite recent progress (12) (13) (14) , the exact mechanism by which a downstream RNA structure stimulates À1 frameshifting remains unclear. The current view is that the downstream RNA element forms a physical barrier that causes a fraction of ribosomes to stall in their translocation step and puts tension on the mRNA-tRNA interaction (15) . This tension is relieved by realignment of A-site and P-site tRNAs in the 5 0 -direction, whereafter the ribosome resumes translation in the À1 reading frame. Several data also point to a role for the E-site tRNA in stimulating frameshifting (16, 17) .