The humanZFX, humanZFY, and mouseZfxgenes have CpG islands near their 5′ ends. These islands are typical in that they span about 1.5 kb, contain transcription initiation sites, and encompass some 5′ untranslated exons and introns. However, comparative nucleotide sequencing of these human and mouse islands provided evidence of evolutionary conservation to a degree unprecedented among mammalian 5′ CpG islands. In one stretch of 165 nucleotides containing 19 CpGs, mouseZfxand humanZFXare identical to each other and differ from humanZFYat only 9 nucleotides. In contrast, we found no evidence of homologous CpG islands in the mouseZfygenes, whose transcription is more circumscribed than that of humanZFX, humanZFY, and mouseZfx.Using the isoschizomersHpaII andMspI to examine a highly conserved segment of theZFXCpG island, we detected methylation on inactive mouse X chromosomes but not on inactive human X chromosomes. These observations parallel the previous findings that mouseZfxundergoes X inactivation while humanZFXescapes it.
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