Frameshifting at the mammalian antizyme mRNA shift site, UCC UGA, is stimulated by two cis-acting signals (39, 40) . One of these, the 5 0 element, encompasses $50 bases upstream of shift site and is important for the polyamine effect (39) (40) (41) . The other cis-acting element is a pseudoknot located 3 0 of the shift site. The mammalian antizyme pseudoknot and a structurally distinct counterpart in a subset of invertebrate antizyme mRNAs (31) are the only pseudoknots known to act as stimulators for +1 frameshifting in eukaryotes.