RESUMEN
Accumulation of the Xist long noncoding RNA (lncRNA) on one X chromosome is the trigger for X chromosome inactivation (XCI) in female mammals. Xist expression, which needs to be tightly controlled, involves a cis-acting region, the X-inactivation center (Xic), containing many lncRNA genes that evolved concomitantly to Xist from protein-coding ancestors through pseudogeneization and loss of coding potential. Here, we uncover an essential role for the Xic-linked noncoding gene Ftx in the regulation of Xist expression. We show that Ftx is required in cis to promote Xist transcriptional activation and establishment of XCI. Importantly, we demonstrate that this function depends on Ftx transcription and not on the RNA products. Our findings illustrate the multiplicity of layers operating in the establishment of XCI and highlight the diversity in the modus operandi of the noncoding players.