The epigenetic phenomenon called X chromosome inactivation plays critical roles in female development in eutherian mammals, and has attracted attention in the fields of developmental biology and regenerative biology in efforts to understand the pluripotency of stem cells. X chromosome inactivation is routinely studied after cell fixation, but live imaging is increasingly being required to improve our understanding of the dynamics and kinetics of X chromosome inactivation and reactivation processes. Here, we describe our live imaging method to monitor the epigenetic status of X chromosomes using a gene knock-in mouse strain named "Momiji" and give an overview of the application of this strain as a resource for biological and stem cell research.