Histones and DNA were cross-linked by 1% formaldehyde and extracted using a Magna ChIP G kit (Millipore). The chromatin was then sonicated to an average DNA fragment length of 200-300 bp. Illumina sequencing libraries were generated according to the manufacturer’s instructions. Briefly, 5-50 ng of ChIP product was end repaired, a-tailed and ligated to Illumina single end adapters by using an NEB Next kit (New England Biolabs). Adaptor-ligated DNA with a size of 200-300 bp was purified by running a gel and enriched via 19 cycles of PCR amplification with Phusion High-Fidelity DNA Polymerase, followed by sequencing on an Illumina HiSeq 2000 system.