Diagram of the location of introns and exons within a gene.

Exons are the regions of DNA within a gene that are not spliced out from the transcribed RNA and are retained in the final messenger RNA (mRNA) molecule. Exons of many eukaryotic genes are interrupted by segments of non-coding DNA (introns). The term "exon" was coined by Walter Gilbert in 1978.


In many genes, each exon contains part of the open reading frame (ORF) that codes for a specific portion of the complete protein, however, the term exon is often misused to refer only to coding sequences for the final protein. This is not true, since many noncoding exons are known in human genes (Zhang 1998).

Some of the exons will be wholly or part of the 5' untranslated region (5' UTR) or the 3' untranslated region (3' UTR) of each transcript. The untranslated regions are important for efficient translation of the transcript as well as being important for controlling the rate of translation and half life of the transcript. Furthermore, transcripts made from the same gene may not have the same exon structure since parts of the mRNA could be removed by the process of alternative splicing. Some mRNA transcripts have exons with no ORF's and thus are sometimes referred to as non-coding RNA.

Exonization is the creation of a new exon, as result of mutations in intronic sequences.

Polycistronic messages have multiple ORF's in one transcript and also have small regions of untranslated sequence between each ORF.


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