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X chromosome inactivation (XCI) is a mechanism specifically initiated in female cells to silence one X chromosome, thereby equalizing the dose of X-linked gene products between male and female cells. XCI is regulated by a locus on the X chromosome termed the X-inactivation centre (XIC). Located within the XIC is  XIST , which acts as a master regulator of XCI. During XCI,  XIST  is upregulated on the inactive X chromosome and chromosome-wide  cis  spreading of  XIST  leads to inactivation. In mouse, the Xic comprises  Xist  and all  cis -regulatory elements and genes involved in  Xist  regulation. The activity of the XIC is regulated by  trans -acting factors located elsewhere in the genome: X-encoded XCI activators positively regulating XCI, and autosomally encoded XCI inhibitors providing the threshold for XCI initiation. Whether human XCI is regulated through a similar mechanism, involving  trans -regulatory factors acting on the XIC has remained elusive so far. Here, we describe a female individual with ovarian dysgenesis and a small X chromosomal deletion of the XIC. SNP-array and targeted locus amplification (TLA) analysis defined the deletion to a 1.28 megabase region, including  XIST  and all elements and genes that perform  cis -regulatory functions in mouse XCI. Cells carrying this deletion still initiate XCI on the unaffected X chromosome, indicating that XCI can be initiated in the presence of only one XIC. Our results indicate that the  trans -acting factors required for XCI initiation are located outside the deletion, providing evidence that the regulatory mechanisms of XCI are conserved between mouse and human.  This article is part of the themed issue ‘X-chromosome inactivation: a tribute to Mary Lyon’.

X chromosome inactivation in a female carrier of a 1.28 Mb deletion encompassing the human X inactivation centre