from Marnie E. Blewitt and Linden J. Gearing writing in Epigenetics: A Reference Manual:
X chromosome inactivation is the method of dosage compensation that has evolved to equalise expression of X-linked genes between female (XX) and male (XY) mammals. In somatic cells only one X chromosome is active; the second X in female cells is silenced early during embryonic development, a process that involves the co-ordination of multiple levels of epigenetic regulation to ensure stable chromosome-wide silencing. In this chapter we shall focus on the molecular mechanisms involved in X chromosome inactivation, discuss how the epigenetic marks are believed to elicit stable transcriptional silencing, and why X inactivation represents an excellent model system for studying epigenetic regulation in mammals.
Further reading: Epigenetics: A Reference Manual