How to inactivate an X-chromosome?

Females have two X chromosomes whereas males have an X and a Y chromosome. In order to prevent an imbalance from occurring during gene expression between sexes, a process known as X chromosome inactivation occurs during embryonic cell development in females. The mechanism of this process was not clearly understood in the past, although the molecule responsible for it was identified to be Xist; a long non-coding RNA .

In every cell that has two X chromosomes, the Xist gene expresses long non-coding RNA which silences almost every single gene on one of the chromosomes. Dossin et al. (link to the paper is mentioned below) recently reported that Xist is able to achieve such a fascinating function by teaming up with an RNA binding protein called SPEN, thereby signaling multiple silencing proteins to the inactivation sites.

However, it is unknown why the Xist gene itself is not silenced by Xist-SPEN complex and other interacting proteins.