Cells were collected and fixed in 1% formaldehyde solution and quenched by 0.125M glycine. Fixed cells were fragmented to a size range of 200-500bp by using Bioruptor Pico. Solubilized fragmented chromatin was immunoprecipitated with antibodies.Fragmented DNA was extracted with phenol-chloroform and precipitated with ethanol DNA fragments acquired from immunoprecipitation would be subjected to end-repaired, adaptor ligation and PCR amplification under the instruction of the manufacturers (New England Biolabs E7370 ).