In female mammals, all X chromosomes except one are transcriptionally inactivated early in embryonic development. This is known as X-chromosome inactivation and is a form of dosage compensation, giving equal dosage of the products of X-linked genes in males and females. The mechanism is of considerable interest as an example of differential behavior of homologous chromatin within the same cell. The system is controlled by the X-inactivation center, a complex locus on the X chromosome, and the key gene is termed Xist. The activity of Xist is controlled by various untranslated RNAs, but polycomb proteins and pluripotency factors play a role at specific stages of embryonic development. At least one gene is thought to be involved in counting X chromosomes and ensuring that a single one remains active in every cell. Some results of recent research are summarized.