Sequence conservation of different groups in human. The graphs show the conservation curve of human adjacent alternative exons around the donor and acceptor sites, based on the alignment between human and mouse (human-referenced, see methods). The x-axis indicates the positions of nucleotides and the y-axis is the average percent identity in a 9-base sliding window. The range of x is 50 nt into the exon and 150 nt into the intron. Subfigure (A) and (B) are for upstream and downstream exons, respectively. Exons in LNK are most conserved, and ME exons are more conserved than IND. The flanking introns in the strongly-correlated groups (ME and LNK), especially the intermediate intron (the intron between upstream and downstream exons), are more conserved than in IND. Of the ME and LNK groups, the intermediate introns in ME show even more conservation. This observation indicates the strongly-correlated groups may be under more delicate regulatory control. Another observation is that the downstream exons are more conserved than upstream ones, especially in the ME group. This reflects the different age of upstream and downstream alternative exons (See text).