exon

An exon is a piece of a gene that makes the final cut — it stays in the mature message after editing and goes on to be read. If introns are the scenes left out, exons are the scenes that end up in the finished film.

After a gene is transcribed, splicing removes the introns and joins the exons end to end. The joined exons form the mature mRNA, whose protein-coding portion the ribosome translates. Exons therefore usually carry the instructions for the protein's sequence, though the very ends of the message can include exon regions that are kept but not translated.

The word can be slightly counterintuitive: an exon is simply what is expressed in the final RNA, not necessarily a tidy single block of code. Because exons can be combined in different ways through alternative splicing, the set of exons a cell chooses helps decide which version of a protein it builds.