Xist (X-inactive specific transcript) is an
RNA gene on the
X chromosome of the
placental mammals that acts as a major effector of the
X inactivation process. It is a component of the
Xic - X-chromosome inactivation centre - along with two other RNA genes (
Jpx and
Ftx) and two
protein genes (
Tsx and
Cnbp2). The Xist RNA, a large (17 kb in humans) transcript, is expressed on the inactive chromosome and not on the active one. It is processed in a similar way 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 chromosome is coated with this transcript, which is essential for the inactivation. X chromosomes lacking Xist will not be inactivated, while duplication of the Xist gene on another chromosome causes inactivation of that chromosome.