Three cis-acting RNA elements are known to stimulate mammalian antizyme 1 frameshifting. One is a 50 nucleotide sequence immediately 5¢ of the shift site (Matsufuji et al., 1995; our unpublished results) . A second stimulator is the UGA stop codon of ORF1 and the third is an RNA pseudoknot starting 3 nucleotides 3¢ of the UGA stop codon. Among frameshift sites of the previously identi®ed antizymes from mammals all the way to Drosophila, there is substantial similarity in the sequences immediately 5¢ of the shift site. Sixteen of the last 18 nucleotides of ORF1 are completely conserved in these genes. Schizosaccharomyces pombe and C.elegans antizymes have 9 of 9 and 6 of 9 (14 out of 19 in O.volvulus) nucleotides identical to the consensus, respectively. For the 5¢ sequences, generally, the more distantly related two antizymes are, the more the similarity is con®ned to the 3¢ end of that region. Our SPA ORF1 deletion data show that mutation of nucleotides that are part of the 5¢ consensus sequence leads to reduced frameshifting ef®ciency. This is another indication that conservation of nucleotide sequence in this region is because of its importance for stimulating ef®cient +1 frameshifting. It is quite striking that in all antizyme gene sequences identi®ed so far, including a number of unpublished ones, ORF1 ends with a UGA stop codon. This is particularly surprising since any of the other two stop codons can substitute for UGA to stimulate antizyme 1 frameshifting, although slightly less ef®ciently, in vitro (Matsufuji et al., 1995) and in vivo (our unpublished results).