The 3¢ pseudoknot that stimulates frameshifting in antizyme 1 is highly conserved in all known vertebrate antizymes, including mammalian antizyme 2 ( Figure 1B) . None of the invertebrate antizyme mRNAs identi®ed so far, including those presented here, has a sequence in the equivalent region that can be simply folded to a comparable RNA structure. However, sequences immediately 3¢ of the frameshift site are conserved between invertebrates and vertebrates. The conservation of this region between Drosophila and the vertebrate counterparts has already been noted (Ivanov et al., 1998b) . The C.elegans antizyme gene contains the sequence YGYCCCYCA (Y = pyrimidine) in this region, which is identical to the consensus. The antizyme genes from the other four nematodes also have a similar sequence ( Figure 1B) . The signi®cance of this similarity is not clear [in fact, sequences in this region appear to play no role in antizyme 1 in vitro frameshifting outside of the RNA pseudoknot context (Matsufuji et al., 1995) ].