In this configuration, the P-and A-site tRNAs can re-pair with at least 2 out of 3 nt when shifted 1 nt towards the 5 0 end of the mRNA. Similarly, for +1 frameshift sites, the identity of the codons in the P-and A-sites of the ribosome is critical for efficient frameshifting. One factor affecting +1 frameshift efficiency is the initial stability of the P-site tRNA-mRNA interaction in the 0 frame (7) . High-efficiency frameshifting occurs when the P-site tRNA does not form standard codon-anticodon interactions (8) . In some studies, a correlation between +1 frameshift efficiency and the final stability of the P-site tRNA-mRNA interaction in the +1 frame has been shown previously (9, 10) . However, in other systems there appears to be little correlation (11) . In addition, competition between decoding of the 0 frame and +1 frame codons in the A-site may affect frameshifting efficiency (7) . Slow to decode 0 frame codons such as stop codons or those decoded by low abundance tRNAs favor frameshifting, as do +1 frame codons with high levels of corresponding cognate tRNAs (12) (13) (14) (15) (16) .