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J. J. Acknowledgements, MRC (MR/K011022/1) grant and postdoctoral fellowship from the Japan Society for the Promotion of Science (JSPS). A.S. is funded from a Wellcome Trust (101050/Z/13/Z) and MRC (MR/K011022/1) grant. All members of the Gurdon group acknowledge the core support provided by the Gurdon Institute core grant from Cancer Research UK, M.O is funded from a Wellcome Trust, 101050.

/. A. , are supported by Cancer Research UK. The Fusion Lumos Orbitrap mass spectrometer was purchased with the support from a Wellcome Trust Multi-user Equipment Grant (Grant #108467/Z/15/Z), Dr Robert Hock (anti-XHMGN2 antibody) and Dr Thomas Jenuwein, pp.3-27

:. J. Conceptualisation, J. J. , M. O. , M. T. , A. S. et al., supervision: J.J