cassette exons

A cassette exon is defined as an exon that is present in one mRNA transcript but absent in an isoform of the transcript.

1. Initial cassette exons are missing in one or more transcript. An initial exon is the 5' exon of a transcript. To be flagged as an initial cassette exon, the exon cannot occur as an internal exon in any transcript, that is, the initial cassette exon is the first exon in the transcript.

2. Terminal casette exons are the same as initial cassette exon, except that they occurs at the 3' end, that is, the terminal cassette exon is the final exon in the transcript.

3. Internal cassette exons are present as an internal exon in at least one transcript of the cluster, that is, neither first nor final. Internal cassette exons are presumed to be the most biologically relevant because truncated sequences may create artificial occurences of initital and terminal cassette exons.