Why aren't 'exons' named 'introns'? Why are introns called 'introns' when they are the actual ones who are getting spliced out from the pre-mRNA. Shouldn't exons be named introns as they are the ones that are 'in' and are not 'exiting'? On what criteria were they named like this?

The terms _intron_ and _exon_ were coined by Walter Gilbert in a renowned 'News and Views' article, Why Genes in Pieces, published in the journal _Nature_ in 1978.

Introns are the _intragenic_ regions and exons are the regions which are _expressed_.

This is the relevant passage in full:

> The notion of the cistron, the genetic unit of function that one thought corresponded to a polypeptide chain, now must be replaced by that of a transcription unit containing regions which will be lost from the mature messenger - which I suggest we call introns (for intragenic regions) - alternating with regions which will be expressed - exons. The gene is a mosaic: expressed sequences held in a matrix of silent DNA, an intronic matrix.