A common mechanism for frameshifting is re-pairing of the peptidyl tRNA in the new reading frame. However, an alternative mechanism whereby the peptidyl tRNA merely occludes the ®rst base of the next codon, has been documented for yeast Ty3 frameshifting (Farabaugh et al., 1993) . Results of experiments with some mutants of the mammalian antizyme 1 shift site pointed to an occlusion mechanism (Matsufuji et al., 1995) . However, the mechanism with the wild-type, UCC-UGA, shift site is not clear. For C.elegans antizyme the UUU-UGA sequence would be an obvious candidate for a re-pairing since Phe-tRNA could pair perfectly with UUU in both frames. But with UCC-UGA the Ser-tRNA ®rst reading UCC could at best pair two out of three with CCU. This important problem warrants further investigation.