X chromosome inactivation is a complex epigenetic process that leads to silencing of one of two X chromosomes in somatic cells of female mammals in order to compensate dosage of X-related genes between the sexes. Major role in this process belongs to Xic region of X chromosome. It contains the XIST gene from which the main signal that drives silencing is transcribed - noncoding Xist RNA transcript. Implementation of silencing occurs in several steps and involves the mutual interaction of non-coding antisense RNA Xist and Tsix, DNA methylation, histone modifications, and other mechanisms for regulation of transcription. The result is condensation of the X chromosome and formation of compact chromatin structure that is maintained through further cell division. Two forms of X chromosome inactivation are random inactivation and imprinted inactivation. In the imprinted mode the X chromosome inherited from the father is predisposed for silencing, while in the random mode mechanism of counting and choice randomly selects either the maternal or paternal chromosome for inactivation. Specifically, by using a blocking factor on one of the X chromosomes, it prevents its inactivation, while the remaining X chromosome(s) inactivate in order to achieve a ratio of one active X per diploid set of autosomes in cell. Imprinted inactivation occurs in marsupials and extra-embryonic placental mammalian tissues, whereas in cells of placental mammalian embryos random inactivation occurs.