To identify the sequence(s) responsible for the enhancement of À1 PRF in vivo, a deletion analysis was carried out. The 5 0 -or the 3 0 -sequences flanking the A-region were deleted from pAC-B/Bm or pAC-C/Cm as indicated in Figure 1B , which generated vectors pAC-AB/ABm, pAC-BA/BAm, pAC-AC/ ACm and pAC-CA/CAm. À1 PRF frequencies were determined in yeast ( Figure 2C ). Increased À1 PRF was observed in all deletion constructs in comparison to the À1 PRF directed by the A region. The BA and AB regions promoted À1 PRF as efficiently as the B region, whereas regions CA and AC were better than region A, but not as good as region B. In other words, the presence of nucleotides 1386-1720, or downstream nucleotides 1602-2137, was sufficient to increase À1 PRF to the level directed by the region B. Thus, the deletion analysis did not identify single specific sequence region as being responsible for the increased À1 PRF frequencies.