X chromosome inactivation: recent advances and a look forward

X chromosome inactivation, the transcriptional inactivation of one X chromosome in somatic cells of female mammals, has revealed important advances in our understanding of development, epigenetic control, and RNA biology. Most of this knowledge comes from extensive studies in the mouse; however, there are some significant differences when compared to human biology. This is especially true in pluripotent cell types and, over the past few years, a significant amount of work has been dedicated to understanding these differences. This review focuses specifically on recent advances in the mechanism of Xist spreading, the role of Xist in cancer, the effects of reprogramming on X chromosome inactivation in human induced pluripotent stem cells, and new tools for studying X chromosome inactivation.