library_construction_protocol: 3–5×107 cells were crosslinked with 1% paraformaldehyde at room temperature for 15 min and sonicated to generate chromatin fragments of 200–600bp. Fragmented chromatin was then immunoprecipitated overnight with one microgram of H3K4me3 antibody (#ab8580; Abcam) incubated overnight at 4 °C. Enriched DNA was isolated by Dynabeads Protein A (#10001D, Thermo Fisher Scientific) and subsequent reverse cross-linking and purification. ChIP–seq libraries were prepared from 1–5 ng ChIP DNA according to the instructions of the TruSeq ChIP Library Preparation Kit (#IP-202-1012, Illumina). Libraries were  sequenced on a HiSeq 2500 System as 50-bp single-end runs.