Retention ratios of highly covered Alu-containing exons. Retention ratio for each exon was calculated by the number of expressed sequences that contain the exon as well as both flanking exons, divided by the total number of sequences that contain both flanking exons. Only the 31 exons with 10 or more total sequences that contain both flanking exons were taken for this analysis. Therefore, every exon represents ∼3% of the exons dataset.