Evidence of Xist RNA-independent initiation of mouse imprinted X-chromosome inactivation

Female mammals carry two copies of the X chromosome, compared to the one X and one Y of the male, and to counter the potentially toxic effect of a double-dose of a gene, most of the genes on one X chromosome are silenced. This process of X-chromosome inactivation (XCI) has long been assumed to be triggered by the expression of the RNA gene Xist, specifically the copy on the inactive X chromosome. New work on embryos of mice engineered to contain a defective Xist gene on their future inactive X chromosome reveals a more complicated picture. In fact, silencing of the paternal X chromosome during imprinted XCI can be initiated in the absence of paternal Xist. In the continued absence of Xist the X chromosome eventually reactivates, suggesting that its role may be to stabilize silencing over the long term.