Females carry two X chromosomes, and the genes of one of those chromosomes cannot be expressed during development or myriad problems arise; thus, silencing one of the chromosomes is essential. That same phenomenon has been shown to be necessary for healthy stem cells as well. Reporting in Cell Stem Cell, scientists at the Paris Diderot University, Institut Curie and the Babraham Institute have now learned more about how one of the X chromosomes is silenced during human development.