High levels of frameshifting are often achieved by the stimulatory action of a cis-acting element located downstream of the shift site. A wide variety of structures, most commonly H-type pseudoknots (17) , have been identified which stimulate À1 frameshifting in eukaryotes [for reviews see (18, 19) ]. Mutagenic and structural data for several of the frameshift stimulators have demonstrated that each pseudoknot has key structural features required for frameshift stimulation (20) (21) (22) (23) (24) (25) (26) (27) (28) . However, unifying structural feature essential for frameshifting has not yet been identified. This observation combined with recent reports that simple antisense oligonucleotides can functionally mimic cis-acting 3 0 stimulators of À1 frameshifting (29, 30) demonstrates that many different structures can stimulate frameshifting. Although it should be noted that not all structures of equal thermodynamic stability can stimulate frameshifting (Discussion). RNA pseudoknots have also been shown to stimulate programmed +1 frameshifting in many eukaryotic antizyme genes (31, 32) . Antizyme is a negative regulator of cellular polyamine levels through its ability to target ornithine decarboxylase (the rate-limiting enzyme in polyamine biosynthesis) for degradation (33) (34) (35) , inhibits polyamine import (36, 37) and stimulates export (38) . Antizyme expression is induced by high-intracellular polyamine levels, and decreased with lowered levels. The polyamine sensor is a programmed +1 frameshift event that is required for antizyme synthesis. *To whom correspondence should be addressed. Tel: +1 801 585 1927; Fax: +1 801 585 3910; Email: mhoward@genetics.utah.edu Ó 2006 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.