Rsx, a metatherian RNA with Xist-like properties

In female (XX) mammals one of the two X chromosomes is inactivated to ensure an equal dose of X-linked genes with males (XY)(1). X-inactivation in eutherian mammals is mediated by the non-coding RNA Xist(2). Xist is not found in metatherians(3) and how X-inactivation is initiated in these mammals has been the subject of speculation for decades(4). Using the marsupial Monodelphis domestica we identify Rsx (RNA-on-the-silent X), an RNA that exhibits properties consistent with a role in X-inactivation. Rsx is a large, repeat-rich RNA that is expressed only in females and is transcribed from, and coats, the inactive X chromosome. In female germ cells, where both X chromosomes are active, Rsx is silenced, linking Rsx expression to X-inactivation and reactivation. Integration of an Rsx transgene on an autosome in mouse embryonic stem cells leads to gene silencing in cis. Our findings permit comparative studies of X-inactivation in mammals and pose questions about the mechanisms by which X-inactivation is achieved in eutherians.