In CfMV, the motif for À1 PRF is the slippery heptamer U UUA AAC and a stem-loop structure 7 nt downstream (25) . The efficiency of À1 PRF directed by CfMV cis-acting signals was assayed in vivo using a dual reporter vector system ( Figure 1A ). Since reporters are produced from one single mRNA, factors that affect the stability of the mRNA as well as the rate of translation initiation have a similar influence on the expression of both reporters, and these variations can be monitored as changes in the activity of the upstream reporter. We quantified À1 PRF by comparing the b-galactosidase normalized firefly luciferase activities derived from the test constructs via À1 PRF to those obtained from the inframe controls, in which identical b-galactosidase-CfMV-firefly luciferase fusions are produced without À1 PRF due to the added nucleotide in front of the slippery heptamer (see Figure 1A ). Similar vectors have been shown to detect even small changes in the recoding efficiencies resulting from alterations in the cis-or trans-acting factors (26, (29) (30) (31) (32) .