Human X chromosome inactivation

Early in a mammalian female's development, one of her two X chromosomes is silenced, thereby achieving dosage equivalence with the single X chromosome found in males. The process of inactivation is initiated by the untranslated XIST RNA that is expressed solely from the inactive X. Subsequent silencing involves the acquisition of many features of heterochromatin, including late DNA replication, DNA hypermethylation, and specific histone modifications. How the XIST RNA associates in cis with the chromosome from which it is expressed and mediates silencing of one X chromosome remains a focus for ongoing research. The inactive X is not completely refractive to transcription, as approximately 15% of human genes escape the silencing and continue to be expressed from both the active and inactive X chromosomes. The hemizygosity of the X chromosome in males means that X-linked diseases are distinctively more common in males, and dosage imbalance of other genes may yield additional differences between the sexes.

Keywords: X chromosome inactivation; XIST; heterochromatin; dosage compensation; DNA methylation; histone modifications; epigenetic silencing