基金Supplementary information is linked to the online version of the paper on the Cell Research website.Acknowledgments We thank Anning Lin (The University of Chicago) for the critical reading of the paper, members in the Chen lab for technical help, the cell biology and molecular biology core facilities for confocal study and Q-PCR, and Shanghai Biochip Co Ltd. for microarray analysis. The H3K27me2 antibody was kindly provided by Li Tang (Fudan University) and Thomas Jenuwein (Research Institute of Molecular Pathology, The Vienna Biocenter). This work was supported by the National Basic Research Program of China (2007CB957900, 2006CB943902, 2007CB947101, 2008KR0695, 2009CB941100, 2005CB522704), the Chinese Academy of Sciences (KSCX2-YW-R-04), the National Natural Science Foundation of China (90919026, 30870538,30623003, 30721065, 30830034, 90919046), the Shanghai Pujiang Program (0757S11361), the Shanghai Key Project of Basic Science Research (06DJ14001, 06DZ22032, 08DJ1400501), and the Council of Shanghai Municipal Government for Science and Technology (088014199).