The discovery of X-chromosome inactivation (XCI) celebrated its golden anniversary this year. Originally offered as an explanation for the establishment of genetic equality between males and females, 50 years on, XCI presents more than a curious gender-based phenomenon that causes silencing of sex chromosomes. How have the mysteries of XCI unfolded? And what general lessons can be extracted? Several of the cell biological mechanisms that are used to establish the inactive X chromosome, including regulatory networks of non-coding RNAs and unusual nuclear dynamics, are now suspected to hold true for processes occurring on a genome-wide scale.