Mechanistic concepts in X inactivation underlying dosage compensation in mammals

Mammals inactivate one of the two female X chromosomes to compensate for the unequal copy number of X-linked genes between males and females. This process of X inactivation entails the silencing of one X chromosome in a developmentally regulated manner. In this work, we review recent findings in X inactivation and discuss how these advance the mechanistic understanding. Recent results provide an insight how the cell counts and chooses the appropriate number of X chromosomes to inactivate, how chromosome-wide gene repression is coordinated and how a stable inactive X chromosome is established. Key components of this complex regulatory system have now been identified and provide entry points for understanding epigenetic regulation in mammals. A majority of the data has been obtained from studying mice. It is presently not clear how general these findings can be applied to other mammalian species. We try to assess this aspect from data, which has become available.