The dual luciferase constructs (0.1 mg) described above were added directly to TNT coupled reticulocyte lysate reactions (Promega) with 35 S-labeled methionine in a volume of 10 ml. Reactions were incubated at 30 C for 1 h. Radiolabeled proteins were separated by SDS-PAGE and the gels were fixed with 7.5% acetic acid and methanol for 20 min. After drying under vacuum, the gels were visualized using a Storm 860 PhosphorImager (Molecular Dynamics) and radioactive bands quantified using ImageQuant software. Percent frameshifting was calculated as the percentage of full-length (frameshift) product relative to the termination product and the full-length product combined. The value of each product was corrected for the number of methionine codons present in the coding sequence. The reported values are the average and standard deviations obtained from at least three independent measurements. Tables showing percent frameshifting and standard deviations can be found in Supplementary Data.