The observation presented here that the antisense oligonucleotide, AZ1A, which anneals directly adjacent to the UGA stop codon can induce ribosome frameshifts to either the +1 or À1 reading frame is surprising. In light of the above discussion of spacing for naturally occurring cis-acting frameshift stimulators, it is possible that frameshifting may occur at codons upstream of the known UCC UGA shift site. However, visual examination of upstream codons does not reveal an obvious À1 or +1 frameshift site.