Figure 1.

Different mechanisms of alternative splicing. Blue boxes indicate exonic regions. Green boxes indicate intronic sequences that are included in mature mRNA by alternative splicing. The upper dashed lines indicate constitutive splicing, whereas the lower dashed lines indicate alternative splicing. (A) Exon skipping (or “cassette exon”) is the most prevalent form of alternative splicing, which involves the complete deletion of one or multiple exons. (B,C) An alternative 3′ or 5′ splice site (ss) can result in a partial intron retention (observed as insertion in the mRNA) or deletion. (D) Intron retention is a less common form of alternative splicing, and involves the inclusion of a complete intron. (E) Introns can include exonic sequences that can contain intact 3′ and 5′ splice sites. The incorporation of these cryptic exons is prohibited during constitutive splicing, but they can be included into the mature mRNA by alternative splicing.