The (epi)genetic control of mammalian X-chromosome inactivation

In mammals, the X chromosome is uniquely capable of complete inactivation. Research in the past two years has validated the long-held hypothesis that the 'X-inactivation center' (Xic) controls events of X inactivation and that its resident gene Xist is not only required but is at least partially responsible for the cis-restriction of X inactivation. Progress has also been made in identifying genes within the Xic. Although Xist remains the only known required element, evidence now suggests that a separate element for X counting must exist and that the Xic may be entirely contained within a 450 kb sequence. This small region may be sufficient for both initiation and establishment of X inactivation.