Equality of the sexes: Mammalian dosage compensation

X chromosome inactivation achieves dosage equivalence for most X-linked gen es between the two X chromosomes in females and the single X chromosome in males. In this article the evidence for random inactivation of an X chromos ome is reviewed, along with the exceptions that result in nonrandom inactiv ation. Another exception to X chromosome inactivation is the presence of ge nes that escape inactivation and are expressed from both the active and ina ctive X chromosomes. The phenotypic consequences of such expression from th e inactive X chromosome are discussed. The major players in the process of inactivation are presented. Initiation of inactivation requires the functio nal RNA, XIST, and the subsequent stable inactivation of the X chromosome r elies upon the recruitment of many other factors, the majority of which are generally associated with heterochromatin.