X-chromosome inactivation (XCI), is the most striking and well-studied example of RME. XCI leads to the transcriptional silencing of one of the X chromosomes in female cells, which results in expression of the genes that located on the other, remaining active X chromosome. XCI is critical for balanced gene expression in female mammals. The allelic choice of XCI by individual cells takes place randomly in epiblasts of the preimplantation embryo, which leads to  mosaic gene expression of the paternal and maternal X chromosome in female tissues. XCI is a chromosome-wide monoallelic expression, that includes expression of all genes that are located on X chromosome, in contrast to autosomal RME (aRME) that relates to single genes that are interspersed over the genome. aRME's can be fixed or dynamic, depending whether or not the allele-specific expression is conserved in daughter cells after mitotic cell division.