DNA methylation of the Xist gene. DNA methylation of the Xist gene. The horizontal line indicates the Xist promoter region. The open rectangle indicates the 5′ portion of the exon 1. The hooked arrow denotes the transcription start site. The methylation-sensitive restriction enzyme sites so far tested are shown: C, HhaI (CfoI); M, MluI; Sn, SnaBI, Sa,SacII; H, HpaII. The methylation status of the methylation-sensitive restriction sites is shown. Open circles indicate lack of methylation; solid circles indicate complete methylation; shaded circles indicate mosaic methylation in that a particular site is methylated with a certain probability on each allele. It is hypothesized that the promoter and the 5′ end of exon 1 are mosaically methylated in eggs. Xi, inactive X chromosome; Xa, active X chromosome. Data for somatic cells and extraembryonic tissues are from reference123; data for ES cells are from references123 and 146; and data for germ cells are from references 4, 123, 124a, 168, and169. Tetsuya Goto, and Marilyn Monk Microbiol. Mol. Biol. Rev. 1998; doi: