Xinactivation

Xinactivation is not a widely defined term in mainstream genetics. When encountered, it may function as a shorthand for either X-inactivation, the process by which one X chromosome is transcriptionally silenced in female mammals, or for X-chromosome reactivation (XCR), the epigenetic reversal of that silencing in certain developmental or experimental contexts. Since the terminology is inconsistent, proper interpretation depends on the source.

X-inactivation occurs in early embryogenesis in female mammals to equalize X-linked gene dosage with males. One X chromosome is randomly chosen per cell and silenced. The long noncoding RNA XIST coats the chosen chromosome, triggering chromatin modifications such as H3K27me3, DNA methylation, and histone deacetylation, leading to transcriptional silencing and late replication. Most genes on the inactive X are silenced, though a subset escapes XCI. The inactivated state is clonally propagated through cell divisions, producing mosaic expression patterns in tissues.

X-chromosome reactivation refers to the re-expression of previously silenced X-linked genes. Natural XCR occurs during germ cell development and in the reprogramming of somatic cells to induced pluripotent stem cells, when the inactive X can be reactivated. XCR involves removal of repressive marks, downregulation of XIST, and changes in chromatin structure. Potential applications include studying dosage compensation and treating X-linked diseases; challenges include incomplete reactivation and epigenetic instability.

Because Xinactivation is not an established term, readers should consult authoritative sources for terminology. See also XIST, XCI, XCR, and dosage compensation.