An exon is a contiguous interval of RNA found in a pre-mRNA molecule. Exons are interspersed with introns, or intervals that are excised during RNA splicing to yield a final molecule of mRNA that is ready for protein translation.

We may also refer to exons of DNA, which are simply the regions of a gene that are transcribed into exons of RNA. The collection of the gene's exons is known as its coding region.

The structure of a gene, broken into introns and exons, is shown below.