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X-Chromosome Inactivation: XCE and the Candidate Region for the X-Inactivation Center

Philip Avner


X-Chromosome inactivation is characterized by the transcriptional silencing of one of the two X chromosomes present in somatic cells of the female. Transcriptional silencing of the X chromosome carried by male germ cells having actively entered meiosis is also observed, but it remains to be proved whether the mechanism underlying X-inactivation in the two cases is identical (Lyon 1983).

The process of X-inactivation has been somewhat arbitarily divided into three phases: initiation, spreading, and maintenance. Most theories of initiation suggest it is triggered by a developmental cue at strictly programmed specific stages of embryogenesis in the female and spermatogenesis in the male. In the female mouse, X-inactivation first occurs in the trophoectoderm and extraembryonic endoderm at 3.5–4.5 days of gestation. This is followed by the X-inactivation of the tissues of the embryo itself, which occurs at the egg cylinder stage, between days 5.5 and 6.0 of gestation, around the time of uterine implantation (for review, see Epstein 1983; Lyon 1983). The developmental cue that triggers inactivation is thought to be associated with the presence of a “blocking signal” that is produced in limited quantity and binds to a single X chromosome per diploid cell, protecting it from inactivation. Implicit in this model is the idea that prior to, or during, the initial phases of inactivation some sort of counting mechanism is capable of sensing the number of X chromosomes in relation to the autosomal chromosome complement. There is no evidence that this “sensing” is necessarily direct, and it has...

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