XIST expression, XIST coating and accumulated markers of heterochromatin on the Xi. It is widely thought that human embryos do not employ XCI prior to implantation. Female embryos have an accumulation of Xist RNA on one of the two X chromosomes, beginning around the 8-cell stage. Xist RNA accumulates at the morula and blastocyst stages and is shown to be associated with transcriptional silencing of the Xist-coated chromosomal region, therefore indicating dosage compensation has occurred. Recently, however, it has become increasingly apparent that XCI of the paternal X chromosome is already present from the 4-cell stage onward in all cells of preimplantation mouse embryos, not the 8-cell stages. Xist, Xite, and Tsix and their roles in X-inactivation Xite and Xist, are both long non-coding RNAs that regulate and facilitate the process of X-inactivation and are important in the silencing of genes within the X chromosome that is being inactivated. These work in combination with Tsix, which is non-coding RNA that is an antisense which downregulates the effects of Xist on the X chromosome in which it is expressed on the maternal X chromosome upon the start regulation of X-inactivation. These three RNAs regulate the X-X pair in a in order to be able to have both chromosomes available for inhibitory actions. Tsix and Xite have basic lncRNAs functions in addition to X-inactivation and regulate the X-X pair in the . This ensures exclusive silencing for both X chromosomes. Xite and Tsix are both essential within the orientational directional processes in cis and trans as it is seen that without Tsix and Xite in trans it perturbs pairing and counting of genes. Once the Xist is turned off and no longer regulates the process, the Tsix will slowly decrease in expression as well until both RNAs are no longer being changed by Xic. Xite is the locus that harbors intergenic transcription start sites from hypersensitive sites of allelic crossovers/differences. When X-inactivation begins, the transcription of Xite increases and signals for the downregulation of Tsix in , which is on the silent X chromosome, all while promoting the Tsix persistence on the active X chromosome. Xite also has major roles to play in the asymmetry of Tsix expression and generates X chromosome inequality through moving and helping orient the chromosomes to be acted upon by the correct subsequent lncRNA, either Tsix or Xist. Neo-sex chromosomes and Dosage compensation The monarch butterfly Danaus