Exon

Eukaryotic genes are usually split into pieces. Exons are the segments that survive RNA splicing and end up joined together in the mature messenger RNA, while the introns between them are cut out. Most exon sequence codes for amino acids, but the first and last exons often include untranslated regions, stretches that help control the RNA's stability and translation instead of coding for protein.

A common mistake is to think introns are the coding parts. It is the exons that are expressed, and the name reflects that: Walter Gilbert coined it in 1978 for the expressed regions of a gene. Because the spliceosome can include or skip certain exons, one gene can produce several related messenger RNAs and proteins, a process called alternative splicing. This helps explain how about 20,000 human protein-coding genes give rise to a far larger variety of proteins.

Exons make up only a small fraction of the human genome. Whole-exome sequencing reads just these regions, a cheaper way than whole-genome sequencing to search for disease-causing changes in the coding parts of genes.