Laboratory of Medical Epigenetics, Institutes of Biomedical Sciences, Medical College of Fudan University & Chinese Academy of Medical Sciences, Shanghai, China. glxu@sibcb.ac.cn. State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Center for Excellence in Molecular Cell Science, Chinese Academy of Sciences, Shanghai, China. glxu@sibcb.ac.cn. International Institute of Molecular and Cell Biology, Warsaw, Poland. mbochtler@iimcb.gov.pl. Polish Academy of Sciences, Institute of Biochemistry and Biophysics, Warsaw, Poland. mbochtler@iimcb.gov.pl.