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-chromosome inactivation in eutherian mammals is mediated by the non-coding RNA Xist2. Xist is not found in metatherians3 (marsupials), and how X-chromosome inactivation is initiated in these mammals has been the subject of speculation for decades4. Using the marsupial Monodelphis domestica, here we identify Rsx (RNA-on-the-silent X), an RNA that has properties consistent with a role in X-chromosome 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, in which both X chromosomes are active, Rsx is silenced, linking Rsx expression to X-chromosome 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-chromosome inactivation in mammals and pose questions about the mechanisms by which X-chromosome inactivation is achieved in eutherians.

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Primary accessions

Gene Expression Omnibus

Data deposits

RNA-seq data is available from the Gene Expression Omnibus under accession number GSE36861; Rsx accession number JQ937282.


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We thank D. Bell and R. Lovell-Badge for advice on the characterisation of Rsx, J. Cloutier and G. Polikiewicz for help with germ-cell preparations and quantitative PCR, the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) Genomics core (National Institutes of Health, NIH) for RNA sequencing, A. Toth for the HORMAD1 antibody, the Biological and Procedural Services units at the National Institute for Medical Research (NIMR) for animal husbandry and Rsx transgenesis, and J. Cocquet, L. Reynard, H. Byers and members of the Turner and P. Burgoyne laboratories for reading of the manuscript. This work was supported by the Medical Research Council (MRC) (U117588498, U117597141, U117581331, U117597137), the NIH (HD60858), the Robert J. Kleberg Jr and Helen C. Kleberg Foundation, the New Zealand Foundation for Research, Science and Technology, Possum Biocontrol (C10X0501), the Australian National Health and Medical Research Council (1010453) and the NIDDK (NIH) Intramural Research Program.

Author information


  1. MRC National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK

    • Jennifer Grant
    • , Shantha K. Mahadevaiah
    • , Mahesh N. Sangrithi
    • , Hélène Royo
    • , Willie Taylor
    • , Greg Elgar
    • , Mike J. Gilchrist
    •  & James M. A. Turner
  2. National Institute of Diabetes, Digestive and Kidney Diseases, NIH, Bethesda, Maryland 20892, USA

    • Pavel Khil
    •  & R. Daniel Camerini-Otero
  3. Landcare Research - Manaaki Whenua, Pest Control Technology Group, Lincoln 7640, New Zealand

    • Janine Duckworth
  4. University of Texas at San Antonio, San Antonio, Texas 78249, USA

    • John R. McCarrey
  5. Texas Biomedical Research Institute, San Antonio, Texas 78227, USA

    • John L. VandeBerg
  6. Department of Zoology, University of Melbourne, Victoria, Australia 3010

    • Marilyn B. Renfree


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J.G. and J.M.A.T. conceived and designed the experiments, performed RNA FISH and RT–PCR and wrote the manuscript. P.K., R.D.C.-O. and M.J.G. generated and analysed RNA-seq data. J.G., G.E. and W.T. performed repeat analysis. J.M.A.T. and S.K.M. performed northern blots. M.N.S. generated the transgenic ES cell line, and H.R. determined the ES cell transgene copy number. J.D., J.R.M., J.L.V. and M.B.R. provided animals and tissues.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to James M. A. Turner.

Supplementary information

PDF files

  1. 1.

    Supplementary Information

    This file contains Supplementary Figures 1-4, a Supplementary Discussion and additional references.

Excel files

  1. 1.

    Supplementary Table

    This file contains the probe and primer sequences used in this study.

  2. 2.

    Supplementary Table 2

    This file contains the female:male ratios for X-encoded transcripts derived from RNA-seq data.

  3. 3.

    Supplementary Table 3

    This file contains the Australasian marsupial Rsx EST identifiers.

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