Cells were lysed at 37°C before chromatin was fragmented, adaptor-ligated and released. For one bio-ChIP experiment, 10 μl streptavidin beads (Invitrogen, cat#60210) were used for incubation for 2 h at 4°C. Beads were briefly washed with 2% SDS/PBS once, and then washed with high salt buffer (1% Triton X-100, 2 mM EDTA, 50 mM HEPES pH 7.9, 500 mM NaCl, 0.1% sodium deoxycholate) for three times. Beads were re-suspended in 100 μl elution buffer (50 mM Tris-HCl pH 8.0, 10 mM EDTA, 1% SDS). After reverse crosslinking and proteinase K digestion, DNA was purified through phenol-chloroform extraction. NEBNext Ultra II Master Mix was used for PCR enrichment, followed by size selection of 200-800 bp. The resulting libraries were subjected to 2 X 150 bp paired-end sequencing on NovaSeq 6000 systems.