Readthrough efficiencies of upstream mutants. The different stop codon context mutations were inserted at the beginning of the luc gene, as illustrated in Figure 1. Each construct was used to transfect NIH3T3 mouse cells and luciferase activity was determined. Each value is the mean of at least 3 independent experiments. Readthrough is expressed as percentage of the corresponding control where the stop codon has been replaced by a sense CAG glutamine codon. A: CAN UAG CAA mutants B: GGN UAG CAA mutants C: Nucleotide usage at the -1 position relative to the stop.