Distribution of SNPs in exons. The graph shows the frequencyN‘_i adjusted for uneven distribution of exon lengths (see Methods). Distances from the 5′ and 3′ exon ends are indicated as positive and negative values, respectively. The increasing trend in the adjusted frequency N‘_i with distance from the exon-intron boundaries implies that the probability of finding an SCE decreases with distance from the splice site but is still nonzero at distances of more than 125 bp.