Fundamental issues related to the regulation of stem cell fate and reprogramming, especially with regard to human cells, remain to be resolved. X chromosome inactivation is one of those important processes of human development related to stem cell biology that we know surprisingly little about. A normal female has two X chromosomes and no Y chromosome and males have one X and one Y chromosome. To be equal with males, females must shut off one of two X chromosomes during embryonic development by inducing the silencing of one of the two X chromosomes (X-inactivation), such that only one X chromosome remains active in every cell of the female body. X-inactivation is an epigenetic phenomenon that occurs without alterations in the primary sequence of DNA by formation of a repressive heterochromatin structure early in development. However, how the initiation of X-inactivation is regulated in human development and how the inactive X chromosome is then stably maintained throughout cell division remains unclear. In this work, we study these questions using human induced pluripotent stem cells (iPSCs) as model system. Since various epigenetic states of the X chromosome have been observed in human iPSCs, our work is also important for the characterization and classification of the epigenetic state of human iPSCs, which has implications for the use of these cells in the laboratory and clinic. During the past funding period, we have worked towards the establishment of the iPSC lines that will be used for the characterization of the X-inactivation process.