Figure 3.

Game of introns. During splicing, introns are excised; however, some introns are retained and are present in the otherwise completely processed and mature RNA. Depending on the features of the retained intron, and on the characteristics of retention (definitive or temporary), the alternative splicing outcome has different fates. Exitrons are reference isoforms that are exported and translated as the (not-so-appropriately-called) “fully spliced”, the commonly coding and fully processed isoforms. Canonical intron retention, where the introns likely include stop codons in their sequence, leads to nuclear retention or to NMD in case of export to the cytoplasm. Intron detention is a specific case of intron retention that is temporary, as the splicing resumes (post-transcriptionally) after a signal or activation of the cell. See the text for further details.