Intron

Introns are sections of DNA within a gene that do not encode part of the protein that the gene produces, and are spliced out of the mRNA that is transcribed from the gene before it is exported from the cell nucleus. Introns exist mainly (but not only) in eukaryotic cells. The regions of a gene that remain in the spliced mRNA are called exons.

Introns sometimes allow for alternative splicing of a gene, so that several different proteins that share some sections in common can be produced from a single gene. The control of mRNA splicing, and hence of which alternative is produced, is performed by a wide variety of signal molecules.

Introns also sometimes contain "old code," sections of a gene that were probably once translated into protein but which are now discarded.

Some introns are actually ribozymes that are capable of catalyzing their own splicing out of the primary RNA transcript. They remove themselves on their own.

The amount of intron DNA varies widely between species. The pufferfish species Fugu rubripes has a very low amount of intron DNA (and its genome was sequenced for this reason), whereas related species have higher amounts.

Introns are not be confused with junk DNA, which is all DNA without known function that is not part of a gene.