Junk DNA: Noncoding regions of DNA that have no apparent function. The term "junk DNA" is a disparaging one, expressing some of the disappointment felt by geneticists when they first gazed upon sizable segments of the genetic code and, instead of seeing one wonderful gene after another, they saw a few exons surrounded by vast stretches of "junk DNA."
Exons are the regions of DNA that contain the code for producing the polypeptide molecules that make up protein. Each exon codes for a specific portion of the complete protein. In humans and some other species, the exons are separated by long regions of junk DNA.
However, junk DNA has been found to be even more conserved than protein-coding regions of the DNA in humans and other mammalian species. The extent of conservation indicates that there is some function for junk DNA that remains to be determined. Junk DNA may prove not to be junk.