Xist (X-inactive-specific transcript) is a long ncRNA gene on the X chromosome of the placental mammals that acts as major effector of the X chromosome inactivation process forming Barr bodies.
X-inactivation, also called lyonization, a process by which one of the two copies of the x chromosome present in female mammals is inactivated
In 1959 Susumu Ohno showed that the two X-chromosomes of mammals were different: one appeared like the autosomes; the other was condensed and heterochromatic.