X-Chromosome Kiss and Tell: How the Xs Go Their Separate Ways

Abstract

Loci associated with noncoding RNAs have important roles in X-chromosome inactivation (XCI), the dosage compensationmechanism by which one of two X chromosomes in female cells becomes transcriptionally silenced. The Xs start out asepigenetically equivalent chromosomes, but XCI requires a cell to treat two identical X chromosomes in completely differentways: One X chromosome must remain transcriptionally active while the other becomes repressed. In the embryo of eutherianmammals, the choice to inactivate the maternal or paternal X chromosome is random. The fact that the Xs always adoptopposite fates hints at the existence of a trans-sensing mechanism to ensure the mutually exclusive silencing of one of thetwo Xs. This paper highlights recent evidence supporting a model for mutually exclusive choice that involves homologouschromosome pairing and the placement of asymmetric chromatin marks on the two Xs.