About 45 percent of human DNA lies between transcription units. Much of this sequence is not repeated anywhere else in the genome. Enhancers on the order of 50–200 bp in length that help to regulate transcription at distant promoters occur in these long stretches of intergenic DNA, as well as in introns. These enhancers (discussed in Chapter 9) are often conserved during evolution, while the neighboring intergenic sequences are not conserved. Other conserved intergenic regions may perform significant functions that are not yet understood. For example, they may contribute to the structures of chromosomes discussed in Section 8.6.