Three inserts of varied lengths from the CfMV polyproteinencoding region (ORF2A/2B) were introduced between the two reporters ( Figure 1B) . The A-region, which at 119 bp was the shortest, represented approximately the minimal frameshift signal proven to be functional in vitro (18) . The longest region was the B-insert. At 752 bp, it started from the 5 0 -terminus of the 12 kDa viral genome-linked protein (VPg) gene and continued to the end of ORF2A. This region encodes CfMV protein P27 with an unknown function (28) . Since the minimal requirements for the functional frameshift signal in vivo were not known, an intermediate 349 bp C-sequence was also selected for the analysis. A well-characterized 53 bp frameshift cassette derived from HIV-1 RNA was used as a positive control. Our results regarding the HIV À1 PRF efficiency, 0.7 -0.1% in bacteria, and 4.5 -1.1% in yeast (Figure 2) , are corroborated by those published earlier (26, 33, 34) indicating that our dual reporter system was fully functional.