The inactivated X is called a Barr body. Early in development, both X's are active 2. As development proceeds, one X is turned off in each cell 3. This inactivation is random 4. This process is irreversible See Figure 5.23.
X inactivation occurs at about the time of implantation.
Some genes are not inactivated and are transcribed from both X's. Initiation involves choosing an X to be inactivated. The size of the Xic in humans is between 120 kb and 660 kb. The Xist gene is included in the Xic region of the chromosome. In both humans and mouse, the Xist genes are ubiquitously expressed in female somatic tissues.