It has been noticed that the length of conservations in flanking introns of conserved alternative exons, i.e., exons that are alternative in several species, is greater than the length of conservations in flanking introns of conserved constitutive exons, i.e., exons that are constitutive in several species [26], suggesting that introns carry cis-acting elements that regulate alternative splicing. In fact, short cis-acting motifs that are necessary for binding splicing factors have been recognized and named intronic splicing silencers and intronic splicing enhancers.
