Intergenic regions may contain a number of functional DNA sequences such as promoters and regulatory elements, enhancers, spacers, and (in eukaryotes) centromeres.
[2] It is possible that these regions contain as of yet unidentified functional elements, such as non-coding genes or regulatory sequences.
[3] In humans, intergenic regions comprise about 50% of the genome, whereas this number is much less in bacteria (15%) and yeast (30%).
[5] Functional elements in intergenic regions will evolve slowly because their sequence is maintained by negative selection.
[6][7][verification needed] Junk DNA sequences are not maintained by purifying selection but gain-of-function mutations with deleterious fitness effects can occur.