How various layers of epigenetic repression restrict somatic cell nuclear 
reprogramming is poorly understood. The transfer of mammalian somatic cell 
nuclei into Xenopus oocytes induces transcriptional reprogramming of previously 
repressed genes. Here, we address the mechanisms that restrict reprogramming 
following nuclear transfer by assessing the stability of the inactive X 
chromosome (Xi) in different stages of inactivation. We find that the Xi of 
mouse post-implantation-derived epiblast stem cells (EpiSCs) can be reversed by 
nuclear transfer, while the Xi of differentiated or extraembryonic cells is 
irreversible by nuclear transfer to oocytes. After nuclear transfer, Xist RNA is 
lost from chromatin of the Xi. Most epigenetic marks such as DNA methylation and 
Polycomb-deposited H3K27me3 do not explain the differences between reversible 
and irreversible Xi. Resistance to reprogramming is associated with 
incorporation of the histone variant macroH2A, which is retained on the Xi of 
differentiated cells, but absent from the Xi of EpiSCs. Our results uncover the 
decreased stability of the Xi in EpiSCs, and highlight the importance of 
combinatorial epigenetic repression involving macroH2A in restricting 
transcriptional reprogramming by oocytes.