Many noncoding DNA sequences must have some important biological function. This is indicated by comparative genomics studies that report highly conserved regions of noncoding DNA, sometimes on time-scales of hundreds of millions of years. This implies that these noncoding regions are under strong evolutionary pressure and positive selection.[47] For example, in the genomes of humans and mice, which diverged from a common ancestor 65–75 million years ago, protein-coding DNA sequences account for only about 20% of conserved DNA, with the remaining 80% of conserved DNA represented in noncoding regions.[48]