text: Slippery sequences can potentially make the reading ribosome "slip" and skip a number of nucleotides (usually only 1) and read a completely different frame thereafter. In programmed −1 ribosomal frameshifting, the slippery sequence fits a X_XXY_YYH motif, where XXX is any three identical nucleotides (though some exceptions occur), YYY typically represents UUU or AAA, and H is A, C or U. In the case of +1 frameshifting, the slippery sequence contains codons for which the corresponding tRNA is more rare, and the frameshift is favored because the codon in the new frame has a more common associated tRNA. One example of a slippery sequence is the polyA on mRNA, which is known to induce ribosome slippage even in the absence of any other elements.