A large family of ancient repeat elements in the human genome is under strong selection.

Although conserved noncoding elements (CNEs) constitute the majority of sequences under purifying selection in the human genome, they remain poorly understood. CNEs seem to be largely unique, with no large families of similar elements reported to date. Here, we search for CNEs among the ancestral repeat classes in the human genome and report the discovery of a large CNE family containing >900 members. This family belongs to the MER121 class of repeats. Although the MER121 family members show considerable sequence variation among one another, the individual copies show striking conservation in orthologous locations across the human, dog, mouse, and rat genomes. The element is also present and conserved in orthologous locations in the marsupial, but its genome-wide dispersal postdates the divergence from birds. The comparative genomic data indicate that MER121 does not encode a family of either protein-coding or RNA genes. Although the precise function of these elements remains unknown, the evidence suggests that this unusual family may play a cis-regulatory or structural role in mammalian genomes.