Xist

Overview
Xist is an RNA gene on the X chromosome of the placental mammals that acts as major effector of the X inactivation process.

The Xist RNA, a large (18 kb) transcript, is expressed on the inactive chromosome and not on the active one. It is processed similarly to mRNAs, through splicing and polyadenylation, however, it remains untranslated. It has been suggested that this RNA gene evolved at least partly from a protein coding gene that became a pseudogene.

The inactive X is coated with this transcript, which is essential for the inactivation. X lacking Xist will not be inactivated, while duplication of the Xist gene on another chromosome causes inactivation of that chromosome.