Chromatin immunoprecipitation was performed following standard protocol. Basically, 8-10 X10^6 cells were crosslinked with 1% formaldehyde at room temperature for 10 min, and then lysed with RIPA buffer containing 0.3 M NaCl. Specific H3K27me3 antibody was added to the sheared chromatin of 200-700 bp size, and rotated at 4°C for overnight. ChIP'd DNA was purified using PCR Purification Kit (Qiagen) and then quantified by Quant-iT dsDNA HS Assay Kit (Invitrogen). Equal amounts of ChIP enriched DNA (5 ng) under each experimental condition (WT and NULL) were prepared for ChIP-Seq libraries. ThruPLEX-FD Prep Kit (Rubicon Genomics, Cat# R400427) was used to construct the sequencing libraries according to the manufacturer's protocol, and the final products were sequenced on the HiSeq2000.