Figure 1: Five basic modes of alternative splicing. A) Exon skipping: in this case, an exon may be spliced out of the primary transcript or retained. This is the most common mode in mammalian premRNAs. B) Alternative acceptor site: An alternative 3’ splice junction is used, changing the 5’ boundary of the downstream exon. C) Alternative donor site: An alternative 5’ splice junction is used, changing the 3’ boundary of the upstream exon. D) Mutually exclusive exons: One of two exons is retained in mRNAs after splicing, but not both. E) Intron retention: A sequence may be spliced out as an intron or simply retained.