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X chromosome inactivation and active X upregulation in therian mammals: facts, questions, and hypotheses Free
Reiner A. Veitia1,2,*, Frédéric Veyrunes3, Samuel Bottani2,4, and James A. Birchler5
1Institut Jacques Monod, Paris, France
2Université Paris Diderot, Paris, France
3Institut des Sciences de l'Evolution de Montpellier, CNRS/Université Montpellier II, Montpellier, France
4Matière et Systèmes Complexes, Paris, France
5Division of Biological Sciences, University of Missouri, Columbia, MO, USA *Correspondence to:Reiner A. Veitia, E-mail:
J Mol Cell Biol, Volume 7, Issue 1, February 2015, Pages 2-11
Keyword: X inactivation, X chromosome upregulation, monoallelic expression, imprinting, dosage balance

X chromosome inactivation is a mechanism that modulates the expression of X-linked genes in eutherian females (XX). Ohno proposed that to achieve a proper balance between X-linked and autosomal genes, those on the active X should also undergo a 2-fold upregulation. Although some support for Ohno's hypothesis has been provided through the years, recent genomic studies testing this hypothesis have brought contradictory results and fueled debate. Thus far, there are as many results in favor as against Ohno's hypothesis, depending on the nature of the datasets and the various assumptions and thresholds involved in the analyses. However, they have confirmed the importance of dosage balance between X-linked and autosomal genes involved in stoichiometric relationships. These facts as well as questions and hypotheses are discussed below.