In mammals, dosage compensation is achieved by X inactivation 1 and is regulated in cis by the X-inactivation centre 2 (Xic) and Xist (refs 3, 4, 5). The Xic controls X-chromosome counting, choice of X to inactivate and initiation of silencing. Xic action culminates in a change in Xist RNA property from a scarce, unstable RNA (Refs 6, 7) to highly expressed Xist RNA that coats the future inactive X (ref. 8). Deleting a 65-kb region downstream of Xist results in constitutive Xist expression and X inactivation, implying the presence of a cis-regulatory element 9. In this region, we now report the discovery of a gene antisense to Xist. Tsix is a 40-kb RNA originating 15 kb downstream of Xist and transcribed across the Xist locus. Tsix sequence is conserved at the human XIC. Tsix RNA has no conserved ORFs, is seen exclusively in the nucleus and is localized at Xic. Before the onset of X inactivation, Tsix is expressed from both X chromosomes. At the onset of X inactivation, Tsix expression becomes monoallelic, is associated with the future active X and persists until Xist is turned off. Tsix is not found on the inactive X once cells enter the X-inactivation pathway. Tsix has features suggesting a role in regulating the early steps of X inactivation, but not the silencing step.