Exon

• Short definition: An exon is a segment of a gene that is transcribed and retained in the mature RNA after splicing. In protein‑coding genes, exons include coding sequences (CDS) and can also include untranslated regions (UTRs); not all exons are protein‑coding.

Exons vs. introns (quick overview)

• During transcription, the entire gene is copied into a primary transcript (pre‑mRNA) that contains both exons and introns.
• Introns are transcribed but removed by RNA splicing; exons are joined (spliced) together to form the mature mRNA.
• Because UTRs are retained in mature mRNA, they are considered exonic even though they are not translated.

See also

• Intron
• Alternative splicing
• Exome

References

• NHGRI Glossary: Exon and Intron
• Wikipedia: Exon
• Cold Spring Harbor DNA Learning Center: Transcription/translation — Exons and introns
• PMC: Not all exons are protein coding: Addressing a common misconception