X-CHROMOSOME INACTIVATION IN MAMMALS

The inactive X chromosome differs from the active X in a number of way s; some of these, such as allocyclic replication and altered histone a cetylation, are associated with all types of epigenetic silencing, whe reas others, such as DNA methylation, are of more restricted use. Thes e features are acquired progressively by the inactive X after onset of initiation. Initiation of X-inactivation is controlled by the X-inact ivation center (Xic) and influenced by the X chromosome controlling el ement (Xce), which causes primary nonrandom X-inactivation. Other exam ples of nonrandom X-inactivation are also presented in this review. Th e definition of a major role for Xist, a noncoding RNA, in X-inactivat ion has enabled investigation of the mechanism leading to establishmen t of the heterochromatinized X-chromosome and also of the interactions between X-inactivation and imprinting as well as between X-inactivati on and developmental processes in the early embryo.