Protein expression levels for the RPS17B gene containing some of Libri et al.'s  mutant introns. Expression levels are normalized with respect to the internal loading control and plotted as a fraction of the wildtype expression level. Shaded boxes represent the mean value for several different samples and error bars represent +/- 1 standard deviation for these samples. The error bar for the wildtype intron comes from the comparison of two different wildtype samples.