Paused ribosomes can pass the À1 PRF site by À1 frameshifting, resumption of 0-frame translation, or termination (43) . Transient polypeptide intermediates that result from the pausing of ribosomes in the slippery sequences have been observed during IBV and S.cerevisiae L-A virus polyprotein synthesis (12, 13, 43, 44) . A pseudoknot structure formed by IBV mRNA causes a translational pause at fixed position upstream the secondary structure regardless of whether the slippery heptamer is present or absent (12) . Based on the findings of this study, we propose that also here a certain percent of ribosomes stalled at the secondary structure of the frameshift site in our inframe control and test mRNAs in yeast, and this led to the prematurely terminated products observed with the inframe control constructs pAC-Bm and -Cm. Although not unambiguously proven by this study, high frequency of termination of translation especially at the frameshift site of the pAC-Cm mRNA would nicely explain the extremely high calculated À1 PRF efficiency.