Samples were cross-linked with 1% formaldehyde for 10 min at room temperature and quenched with 0.125 M glycine for 5 min. Cross-linked cells were lysed. Endogenous nuclease was inactivated with 0.3% SDS, and chromatin DNA were digested by 100 U MboI (NEB), end-labeled with biotin-14-dCTP (Invitrogen) and then ligated by 50 U T4 DNA ligase (NEB). After reversing cross-links, ligated DNA was purified using the QIAamp DNA Mini Kit (Qiagen)