The dosage compensation in placental mammals is achieved by silencing of one copy of the X chromosomes in a female cell by a process called X chromosome inactivation (XCI). XCI ensures equal gene dosage for X-linked genes between the two genders. Although the choice of X chromosome to be silenced is random, once the silencing of the X chromosome has been established, this process is highly regulated and maintained throughout subsequent cell divisions. A long non-coding RNA, Xist, and its interacting proteins execute this multistep process, but several of these regulatory proteins remain unidentified. Recent technological advances based on the genetic and proteomics screening have identified several new regulatory factors as well as dissected the molecular details of XCI regulation. Moreover, identification of regulators of XCI offers an opportunity to explore reactivation of the inactive X chromosome (Xi) as a potential therapeutic strategy to treat X-linked diseases, like Rett syndrome. Here, we summarize recent reports that identified new regulatory proteins and RNA species playing a crucial role in Xist localization and spreading, recruitment of silencing machinery to the Xi, Xist interaction with chromatin, and structural organization of the Xi in the nuclei.

X chromosome inactivation (XCI) is a strategy used by mammals to silence genes along one of the two female X chromosomes and equilibrate expression dosage between XY males and XX females. This epigenetically-inherited silencing is established during early embryonic development and maintained thereafter through cell divisions. Seeding of multiple repressive epigenetic marks along the inactive X chromosome (Xi) makes inactivation extremely robust and difficult to reverse upon single genetic perturbations. Reversal of XCI has, however, been observed when somatic cells are reprogrammed towards pluripotency, and in vitro reprogramming techniques have been used in recent years to dissect Xi gene reactivation mechanisms. These studies pave the way for developing novel therapeutic approaches for X-linked diseases. Here, the author reviews Xi reactivation during pluripotent reprogramming of mouse and human somatic cells, highlight recent advances and species-specific differences, and discuss the relevance for human diseases.