The signal that makes a ribosome shift comprises two elements: a slippery sequence, where the ribosome switches the reading frame, and an adjacent stimulatory signal, usually a specific RNA structure. For À1 frameshifting, the slippery sequence usually consists of a heptanucleotide motif X XXY YYZ, where X can be any three identical nucleotides, Y can be three A's or U's, and Z is not G (8) . The slippery sequence has been shown to be shifty on its own in vitro, up to 2%, but is strongly stimulated, up to 40-fold, by the presence of a hairpin, a pseudoknot, a three-way junction (9) or an antisense oligonucleotide (10, 11) , located 5 to 8 nt downstream of the slippery sequence.