In female somatic cells of mammalian species one X chromosome is inactivated to ensure dosage equality of X-encoded genes between females and males, during development and adulthood. X chromosome inactivation (XCI) involves various epigenetic mechanisms, including RNA mediated gene silencing in cis, DNA methylation, and changes in chromatin modifications and composition. XCI therefore provides an attractive paradigm to study epigenetic gene regulation in a more general context. The XCI process starts with counting of the number of X chromosomes present in a nucleus, and initiation of XCI follows if this number exceeds one per diploid genome. Recently, X-encoded RNF12 has been identified as a dose-dependent activator of XCI. In addition, other factors, including the pluripotency factors OCT4, SOX2 and Nanog, have been implicated to play a role in suppression of initiation of XCI. In this review, we highlight and explain these new and old findings in the context of a stochastic model for X chromosome counting and XCI initiation.

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Keywords DNA methylation, DNA replication, DNA transcription, Rnf12, Stochastic, Tsix, X chromosome inactivation, XCI, Xist, Y chromosome, allele, autosomal inheritance, cell nucleus, chromatin assembly and disassembly, chromosome number, chromosome protein, diploidy, down regulation, embryonic stem cell, epigenetics, gene control, gene deletion, gene expression regulation, gene insertion, gene silencing, genetic stability, heterozygote, human, molecular genetics, nonhuman, octamer transcription factor 4, prediction, priority journal, promoter region, protein RNF12, review, single nucleotide polymorphism, sister chromatid, stochastic model, transcription factor NANOG, transcription factor Sox2, transcription initiation, transgene, unclassified drug, upregulation
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Barakat, T.S., Jonkers, I.H., Monkhorst, K., & Gribnau, J.H.. (2010). X-changing information on X inactivation. Experimental Cell Research: emphasizing molecular approaches to cell biology, 316(5), 679–687. doi:10.1016/j.yexcr.2010.01.015