X chromosome inactivation in mammals is an essential epigenetic process which compensates for X chromosome imbalance between sexes. XCI is established early during female development, at peri-implantation stages, and is triggered by the accumulation of the long noncoding RNA XIST, which recruits a plethora of factors leading to transcriptional silencing and chromatin reorganization.
In contrast to mouse, female primate pre-implantation development proceeds in the absence of X-inactivation; yet XIST is expressed. I will discuss the function of XIST at these stages and its contribution to alternative modes of dosage compensation.