design: The formaldehyde cross-linked DNAs were digested with DpnII and treated with biotin, which were ligated to the ends of the fragmented DNA sequences. Cyclization of DNA after end-repair was used to identify the location of the intersecting DNA. After reversing the crosslinking, ligated DNA fragments were fragmented into 300-700 bp sizes, followed by a biotin-streptavidin purification for library construction.