Only two examples are known where RNA elements 3¢ of the frameshift site stimulate +1 frameshifting. One is the RNA pseudoknot of mammalian antizyme 1 and the second is a short RNA sequence immediately following the frameshift site of Ty3 (Farabaugh et al., 1993) . Additional examples would be very helpful in deciphering the role such elements play in the mechanism of +1 frameshifting. It is currently not known how many and which of the invertebrate antizyme genes contain 3¢ frameshift stimulators. The results presented here show that an S.pombe 3¢ stimulator enhances frameshifting 10-fold. This stimulator appears completely different from the 3¢ RNA pseudoknot in vertebrates. Our deletion experiments indicate that none of the predicted RNA structures contained within the minimally required 3¢ region [up to 150±180 nucleotides downstream of the frameshift site ( Figure 5C )] are suf®cient to confer the stimulatory effect. The SPA 3¢ stimulator may act directly through sequence or may have an unusual RNA structure involving non-Watson±Crick base pairing. More detailed mutagenesis combined with phylogenetic analysis would be required to discern the nature of the 3¢ stimulator of SPA.