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).

Abstract

In female mammals, most genes on one X chromosome are silenced as a result of X-chromosome inactivation^{1, 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 systems^{4, 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.

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).

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