library_construction_protocol: First, one million K562 cells were crosslinked with 1% formaldehyde, and lysed to isolate nuclei as described previously (PMID: 31862872). Second, digestion of essDNA regions in the nuclei was performed as follows: nuclei were centrifuge at 4000 rpm for 10 min at room temperature. After removal of supernatant, the pellet was incubated in 320 µl 0.2% SDS buffer for 10 min at 62℃ and quenched with 50 µl of 10% Triton X-100. The nuclei were collected by centrifugation at 4000 rpm at 27℃ for 10 min and washed twice with 500 µl 1x NEBuffer 1.1 (NEB, Ipswich, MA, USA). After aspiration of the supernatant, the nuclei were resuspended in 50 µl 1x NEBuffer 1.1 (NEB) containing different amount of the Nuclease P1 (NEB) (0.01 U, 0.1 U, 1 U, and 10 U) at 37℃ for 10 min, followed by inactivation at 75℃ for 10 min. Third, DNA was isolated from the P1 digested samples as described previously (PMID: 31862872) and purified with 2x volumes of VAHTS DNA Clean Beads (Vazyme, Nanjing, China) to a final volume of 20 µl. The NGS sequencing libraries were constructed with the SSiNGLe-ILM method developed by us previously (PMID: 31862872), using 100 ng of DNA as input.