In stop codon readthrough a stop codon is decoded by a tRNA carrying an unusual amino acid rather than a translational release factor. Specific stimulatory elements downstream to the stop codon regulate this process (5) . Hopping, in which the ribosome stops translation in a particular site of the mRNA and re-start few nucleotides downstream, is a rare event and it has been studied in detail only in the bacteriophage T4 (6) . In programmed frameshifting, ribosomes are induced to shift to an alternative, overlapping reading frame 1 nt 3 0 -wards (+1 frameshifting) or 5 0 -wards (À1 frameshifting) of the mRNA. This process is regulated and its frequency varies in different genes. The ±1 programmed frameshifting has been studied extensively in viruses, retrotransposons and insertion elements for which many cases are documented (7) (8) (9) . Instead, this phenomenon is by far less common in cellular genes. A single case of programmed +1 frameshifting is known in prokaryotes (10, 11) while in eukaryotes, including humans, several genes regulated by this recoding event have been described previously [(4) and references therein]. Compared to +1 frameshifting, À1 frameshifting is less widespread with only two examples in prokaryotes (12) (13) (14) and few others in eukaryotes (15) (16) (17) .