The frameshift ef®ciency of SPA frameshift site is lower than that observed with mammalian antizyme 1 even when both are tested in the same organism (S.pombe) [for the frameshift ef®ciency of antizyme 1 cassette in S.pombe, see Ivanov et al. (1998c) ]. It is possible that the observed ef®ciencies for S.pombe antizyme are arti®cially low because the constructs do not include all the cis-acting stimulatory elements. On the other hand there is no reason why a lower level of frameshifting does not correctly re¯ect the evolved balance with the other characteristics of the complex system such as relative protein stabilities.