X chromosome inactivation ensures equal dosage of X-linked genes between male and female mammals. Two new studies have shown that the initiation of inactivation is preceded by X chromosome pairing; their results implicate this pairing in the choice and counting functions of X chromosome inactivation.

Mammals balance X-linked gene dosage by silencing one X chromosome in female cells, a process that begins with counting the number of X chromosomes. Recent reports suggest homologous X chromosome pairing may be a prerequisite for silencing, providing a basis for counting by ensuring that silencing only occurs in cells with two X chromosomes.

Mammalian cells must count their X chromosomes to determine whether to initiate X chromosome inactivation. A region that may be important for X chromosome counting has been identified, but the puzzle pieces still do not quite fit.

X inactivation studies have been carried out on 22 pairs of female monozygotic twins, one set of female monozygotic triplets, and their mothers and singleton sisters, using the probe M27 beta. Forty-eight per cent of the twins, 55% of their mothers, and 42% of their singleton sisters showed skewed X inactivation. Two of the triplets and their mother had random X inactivation, while the third tr...

A study of X inactivation in 12 female carriers for adrenoleucodystrophy showed no evidence that skewed patterns are related to clinical manifestation. Other possible mechanisms to explain manifestation in females are considered.

It is a well-known fact of popular science that nearly all tortoiseshell cats are female. Underlying this is X-chromosome inactivation, a phenomenon found in all mammals. In the sex-determining system of mammals — in which females carry two X chromosomes and males an X and a Y chromosome — the X chromosome is typically large and carries many genes unconnected with sex, whereas the Y chromosome ...

X inactivation is a chromosome-specific form of genetic regulation in which thousands of genes on one homologue become silenced early in female embryogenesis. Although many aspects of X inactivation are now understood, the spread of the X inactivation signal along the entire length of the chromosome remains enigmatic. Extending the Gartler-Riggs model [Gartler, S. M. & Riggs, A. D. (1983) Annu....

The third X-inactivation meeting 'Fifty years of X-inactivation research', which celebrated the fiftieth anniversary of Mary Lyon's formulation of the X-inactivation hypothesis, was an EMBO workshop held in Oxford, UK, in July 2011. This conference brought together the usual suspects from the field, as well as younger researchers, to discuss recent advances in X-inactivation research. Here, we ...