library_construction_protocol: The 4C-seq referred to a previous publication. Briefly, the cells were first treated properly and then crosslinked with 1% formaldehyde for 10 min at room temperature. The nuclei were isolated and treated with 25 μl 10×cutsmart buffer and 100 U NlaIII (NEB, #R0125S), incubated overnight at 37℃, 900 rpm to digest the chromatin. The next day, the restriction enzyme was inactivated by heat. The proximal, digested chromatin fragments were ligated by T4 DNA ligase (NEB, M0202) by incubating the reaction for at least 6 h at room temperature. After ligation, the DNA was reverse crosslinked and extracted by an equal volume of phenol:chloroform:isoamyl alcohol (Solarbio, P1013). The purified DNA was fragmented by sonication with Biorupter at high energy 30s on, 60s off, for 2 cycles. The fragmented DNA was examined by 1% DNA agarose gel electrophoresis. The size of DNA product was about 0.2 kb-2 kb. A biotin-labeled primer targeting the region of interest was used for primer extension. The amplified DNA was purified by biotin-streptavidin pulldown. The biotin-labeled single strand DNA was ligated to an adapter by bridge linker-mediated ligation. The ligated DNA products were amplified by nest PCR, and 500 bp-900 bp DNA products were purified by Magen gel purification kit.