RESEARCH HIGHLIGHTS Nature Reviews Genetics | AOP, published online 2 July 2013; doi:10.1038/nrg3538 Photodisc/Getty differentiation in female cells, CTCF binding at this site is depleted in the future inactive X chromosome. Furthermore, CTCF transgene studies indicated that CTCF was a direct Xist repressor. Sun et al. were then able to show a link between Jpx and CTCF: Jpx EPIGENETICS overexpression relieved the effects of CTCF overexpression. Furthermore, X inactivation by titration they showed direct binding of Jpx to CTCF in vivo. Finally, ChIP confirmed X-chromosome inactivation (XCI) Sun et al. postulated that the that when CTCF was overexpressed, in mammals compensates for the X-encoded lncRNA Jpx, which is known the binding of CTCF to P2 was imbalance in X chromosome number to activate the transcription of Xist, is increased, and Xist upregulation between females (XX) and males (XY). a good candidate for an X-encoded was compromised. However, when How the number of X chromosomes is numerator. In support of this hypothesis, Jpx alone was overexpressed, counted by the cell is a long-standing transgene experiments in mouse CTCF binding to Xist was reduced problem. A recent study has implicated embryonic stem cells (ESCs) showed approximately by half. an X-encoded long non-coding that Xist displays a dosage-dependent The authors’ data point towards a RNA (lncRNA) in X-chromosome response to Jpx expression, so Xist model in which, in order to initiate XCI, counting that titrates an autosomal expression is switched on only when the Jpx is activated and titrates repressive transcription factor away from the X levels of Jpx are high enough. CTCF away from the Xist promoter chromosome to inactivate it. The authors then chose to investigate through competitive binding; this A recent study Mammalian XCI is known to occur CTCF as a potential autosomal is the first time that such a function has implicated when the X to autosome ratio (X/A) is regulator of XCI initiation — that is, as has been reported for an lncRNA. ≥1, leading to the expression of inactive an autosome-encoded denominator The allele-specific nature of these an X-encoded X specific transcript (Xist), which is an of the X/A ratio — as putative binding interactions suggests that further long non- lncRNA that silences the X chromosome. sites for the CTCF transcription factor regulatory factors are yet to be found. coding RNA in However, a full understanding of had previously been identified in the Hannah Stower how the cell calculates the X/A ratio Xist promoter. They used chromatin X-chromosome through X-encoded numerators and immunoprecipitation (ChIP) to show ORIGINAL RESEARCH PAPER Sun, S. et al. Jpx counting autosomally encoded denominators has that CTCF binds to the promoter site RNA activates Xist by evicting CTCF. Cell 153, been elusive. P2 in ESCs prior to XCI. However, on 1537–1551 (2013) NATURE REVIEWS | GENETICS VOLUME 14 | AUGUST 2013 © 2013 Macmillan Publishers Limited. All rights reserved.