Cells were fixed with fresh 1% formaldehyde for 10 min at room temperature and 125 mM glycine was added to inactivate the fixation. Cells were washed by cold PBS 3 times and collected. Cross-linked chromatin was sonicated into fragments of a size range between 250 and 500 nucleotides (Covaris). H3K27Ac antibody (Abcam, ab4729) was incubated with the solubilized DNA fragments at 4°C overnight. Antibody–chromatin complexes were captured by protein A/G Agarose beads (Pierce #20423) and eluted with 1% SDS after extensive washing. The cross-link between DNA and chromatin proteins was reversed by incubation overnight at 65°C. DNA was purified by QIAquick PCR Purification Kit (Qiagen 28104) and dissolved into 30 μL TE buffer per immunoprecipitation. ChIP-seq libraries were generated using KAPA LTP library preparation kit(KK8230) following the manufacturer's protocol (Kapa Biosystems).