The X chromosome of marsupials shares a highly conserved region with eutherians.

Ten genes, located on the long arm of the human X chromosome, were mapped in several marsupial species by somatic cell analysis and in situ hybridization. All were located on the X chromosome in each species. We conclude that the long arm of the human X chromosome represents a highly conserved region that formed part of the X chromosome in a therian ancestor 120-150 million years ago, before the mammalian infraclasses diverged.