This leads to dosage compensation problems: the two X chromosomes in the female will create twice as many gene products as the one X in the male.

The inactive X chromosome in cells of females is visible as a Barr body under the microscope.

Xist inactivates an X chromosome at random in female mice by condensing the chromatin, via histone methylation among other mechanisms that are currently being studied.

[4] Tsix and Xist regulate X chromosome protein production in female mice to prevent early embryonic mortality.

However, if the paternal Tsix allele is active, it can rescue female embryos from the over-accumulation of Xist.

[8] When one allele of Tsix in mice is null, the inactivation is skewed toward the mutant X chromosome.

This is due to an accumulation of Xist that is not countered by Tsix, and causes the mutant chromosome to be inactivated.

When both alleles of Tsix are null (homozygous mutant), the results are low fertility, lower proportion of female births and a reversion to random X inactivation rather than gene imprinting.

To confer pluripotency in an embryonic stem cell, factors inhibit Xist transcription.