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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 such as Group I and Group II 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, whereas related species have higher amounts. Introns are not to be confused with junk DNAWithin a chromosome or a genome, the junk" DNA are those portions of the DNA for which no function has been identified. The term "junk" is recognized as something of a misnomer, especially in light of the fact that molecular biology is a young science and, which is all DNA without known function that is not part of a gene.

See also: selfish DNASelfish DNA is DNA which is prevalent in the genome, not because its phenotypic effect is beneficial, but because it has properties which cause the number of copies of it within the genome to increase with time. Examples of selfish DNA are: Transposons co, noncoding DNAIn genetics, noncoding DNA describes DNA which does not contain instructions for making proteins (or other cell products such as RNAs). In eukaryotes, a large percentage of many organism's genomes are noncoding. Some noncoding DNA is involved in regulatin, inteinAn intein is a segment of a protein that is able to excise itself and rejoin the remaining portions (the exteins) with a peptide bond. Inteins have also been called "protein introns". Most reported inteins also contain an endonuclease domain that plays a

Reference: Walter GilbertWalter Gilbert (born March 21, 1932) is an American physicist, biochemist, entrepreneur, and molecular biology pioneer. He was awarded the 1980 Nobel Prize in Chemistry with Paul Berg and Frederick Sanger. Gilbert and Sanger were recognized for their pion (1978 Feb 9) "Why Genes In Pieces?" Nature 271 (5645):501.