X-inactivation

X-inactivation is the epigenetic silencing of one X chromosome in cells of most female mammals. It balances X-linked gene expression between XX and XY cells and creates cellular mosaicism.

Xist: Xist is a long noncoding RNA that coats the X chromosome from which it is transcribed and initiates silencing. It spreads over the future inactive X and recruits silencing machinery.
X chromosome: The X chromosome is a sex chromosome carrying genes involved in many biological functions. Its gene dosage differs between XX and XY cells.
Mary Lyon: Mary Lyon was a British geneticist who proposed random X-chromosome inactivation in female mammals in 1961. Her hypothesis explained coat-color mosaics and established the core model.
Cellular mosaicism: Cellular mosaicism is the presence of genetically or epigenetically distinct cell populations within one organism. Different cells can express genes from different parental X chromosomes.
X-inactivation center: The X-inactivation center is a regulatory region on the X chromosome containing genes and elements involved in initiating inactivation. Its control elements coordinate the chromosome’s silencing decision.
Tsix: Tsix is an antisense long noncoding RNA that represses Xist expression in mouse cells. Its activity helps keep the future active X from expressing Xist.
Sex chromosomes: Sex chromosomes are chromosomes whose combinations contribute to sex determination in many organisms. X-inactivation addresses the unequal number of X chromosomes across sexes.
Murray Barr: Murray Barr was a Canadian physician whose laboratory identified the sex chromatin body in cell nuclei. The condensed inactive X became known as the Barr body.
Skewed X-inactivation: Skewed X-inactivation occurs when one parental X chromosome is inactive in substantially more cells than the other. Unequal cell lineages can make the silencing pattern nonrandom across a tissue.
X-chromosome reactivation: X-chromosome reactivation is the restoration of gene activity on an X chromosome that was previously inactive. Reactivation occurs in specific developmental contexts and complicates the idea of permanent silencing.

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