[X chromosome inactivation in mammals]

The most important results of the last 30 years of studies on mammalian X-chromosome inactivation are reviewed. The data on X-chromosome inactivation in cells of embryonic and extraembryonic tissues and in male and female germ cell lines are discussed. Special attention is paid to data on mapping and functioning of the X-inactivation center and of recently discovered gene XIST. The main hypotheses concerning the mechanisms responsible for X-chromosome inactivation are considered. A new model of X-inactivation is proposed, which regards heterochromatin as a nonspecific activator of nucleation of the X-chromosome on which it is located.