Wild and mutation cells were fixed with 1.0% formaldehyde at room temperature for 15 min, after which glycine was added to the medium. The cells were then lysed and the rabbit polyclonal antibodies against H3K27me3 (#9733, CST) was used for the ChIP assay using the Pierce™ Magnetic ChIP Kit (Thermo, Inc., MA, USA) according to the instructions of manufacturer. Immunoprecipitated DNA (20 ng) was subjected to size selection of fragments around 200-bp in length. Sequencing libraries were consequently constructed by adaptor ligation and PCR. Sequencing was carried out with an Illumina HiSeq2500 system according to the manufacturer's protocol.