Studyofcomparativegenomics has revealed that about 5% of the human genome are underpurifying selection, 3.5% of whichareconservednon-coding elements (CNEs).
Dr Ward and Dr Kellis found that, in addition to the 5% of human DNA that is conserved between mammals, an additional 4% of human DNA appears to be uniquely human in the sense that it is prone to purifyingselection in humans but not in other mammals.