MEFs were cross-linked with 1% formaldehyde solution (Sigma, F8775) for 10 min at room temperature. Fixation was quenched by glycin at a final concentration of 0.125 M for 5 min. Next, chromatin was sheared using a Bioruptor sonicator until DNA was fragmented to 300-500 bp. The supernatant was pre-cleared with 20 μL salmon sperm DNA/Protein A sepharose beads (Sigma, GE17-5280). Subsequently, 250 μg fraction and 3 μg histone modification antibody were combined and agitated overnight at 4 °C. Antibody-protein-DNA complexes were pull-down by 30 μL Protein A sepharose beads slurry and washed. The eluted complexes were subsequently decross-linked at at 65 °C for 5 h. After phenol chloroform extraction step, ChIP DNA were provided for establishing sequencing libraries which followed illumina manufacturer's protocol and the libraries were sequenced on illumina Hiseq platform (Genewiz, Corp). Libraries were prepared for sequencing using standard Illumina protocols