Fixed cells were sonicated using Bioruptor to achieve 200–700 bp size chromatin fragments. Solubilized chromatin was immunoprecipiated with antibody against H3K4me3 (Abcam 8580). Antibody–chromatin complexes were pulled down using Dynabeads protein A (Invitrogen), washed and then eluted. After cross-linking reversal, RNase and proteinase K treatment, immunoprecipiated DNA was purified using AMPure beads (Beckman Coulter). ChIP DNA were end-repaired and 5′ phosphorylated using T4 DNA Polymerase, Klenow and T4 Polynucleotide Kinase (Enzymatics). A single adenine was added to 3′ ends by Klenow (3→5′ exo-), and double-stranded Bioo Illumina Adapters (Bioo Scientific) were ligated to the ends of the ChIP fragments. Adaptor-ligated ChIP DNA fragments were subjected to 15 cycles of PCR amplification using Q5 polymerase (NEB). AMPure beads were used to purify DNA after each step (Beckman Coulter). Pooled libraries were sequenced on the HiSeq2500 (V4 Chemistry) for single-end 50 bp according to the manufacturer’s instructions.