Figure 1.

Types of alternative splicing of a hypothetical gene containing four exons. Exons are shown as colored boxes, whereas introns are shown as solid lines. Dotted lines indicate the joining of exonic sequences by removal of intronic sequences. A. In the absence of alternative splicing all exons are joined by removal of intronic sequences. B. Exon 3 is skipped due to absence of recognition of both, the 3´ and 5´ ss of exon 3. C. Only a portion of exon 4 is included due to activation of an alternative 3´ ss within exon 4. D. Only a portion of exon 2 is included due to activation of an alternative 5´ ss within exon 2. E. An Alu-like sequence is included due to activation of the additional 5´ and 3´ ss within intron 1. F. Intron 2 is retained due to absence of recognition of both, the 5´ ss of exon 2 and the 3´ ss of exon 3. Abbreviations: Int stands for intron, whereas ss stands for splice site.