A second construct, p2luc-AZ1sl, was designed to contain only the 5 0 half of stem 1 of the antizyme pseudoknot downstream from the UCC UGA shift site ( Figure 1A ). 2 0 -O-Methyl antisense oligonucleotides were designed to anneal between 3 and 15 nt (SL1) or 3 and 22 nt (SL2) downstream from the UGA codon of the shift site. Frameshift efficiency induced by these two antisense oligonucleotides, 8 and 22% respectively, was somewhat lower than that observed with PKm1 and PKm2 ( Figure 4C and D and Supplementary Table 4 ). In these cases frameshift efficiency was higher for the longer antisense oligonucleotide (SL2), suggesting that frameshift efficiency most probably correlates with stability of the duplex. As was seen with AZ1A, AZ1B and AZ1C, frameshifting efficiency stimulated by antisense oligonucleotides PKm1, PKm2, SL1 and SL2 was also strongly correlated with the concentration of exogenously added spermidine (Supplementary Table 5) .