Fig. 2. Number of occurrences of each nucleotide in the 3′ sequence. (A) Results of the initial screening. The y-axis represents the number of occurrences at positions +4, +5, +6, +7, +8 and +9. (B) Only sequences giving readthrough values >5% were taken into account. (C) Expected frequencies of nucleotides at +4 to +9 positions from natural stop codons in the yeast genome. Results were obtained from the Transterm database (Jacobs et al., 2000).