Letters to Nature

Nature 434, 400-404 (17 March 2005) | doi:10.1038/nature03479; Received 24 January 2005; Accepted 17 February 2005

X-inactivation profile reveals extensive variability in X-linked gene expression in females

Laura Carrel1 & Huntington F. Willard2

  1. Department of Biochemistry and Molecular Biology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA
  2. Institute for Genome Sciences & Policy, Duke University, Durham, North Carolina 27708, USA

Correspondence to: Laura Carrel1 Correspondence and requests for materials should be addressed to L.C. (Email: lcarrel@psu.edu).

In female mammals, most genes on one X chromosome are silenced as a result of X-chromosome inactivation1, 2. However, some genes escape X-inactivation and are expressed from both the active and inactive X chromosome. Such genes are potential contributors to sexually dimorphic traits, to phenotypic variability among females heterozygous for X-linked conditions, and to clinical abnormalities in patients with abnormal X chromosomes3. Here, we present a comprehensive X-inactivation profile of the human X chromosome, representing an estimated 95% of assayable genes in fibroblast-based test systems4, 5. In total, about 15% of X-linked genes escape inactivation to some degree, and the proportion of genes escaping inactivation differs dramatically between different regions of the X chromosome, reflecting the evolutionary history of the sex chromosomes. An additional 10% of X-linked genes show variable patterns of inactivation and are expressed to different extents from some inactive X chromosomes. This suggests a remarkable and previously unsuspected degree of expression heterogeneity among females.

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