CHIP protocol: Cells were fixed with 1% formaldehyde for 10 min at room temperature. Crossed-linked cells were sonicated at 40% amplitude (30 sec on, 90sec off, for 24 min total) using The Branson 101-135-126 Sonifier. Chromatin IP (ChIP) was performed using monoclonal anti-CSB antibody against the N-terminal 507 amino acids (1B1) [Dr. Hua-Ying Fan, Univ. of Pennsylvania, PMID 23028371]. To generate the library from ChIP experiment, ChIP-Seq Library Protocol with multiplexing was used. This protocol was developed before Illumina released its multiplex truSeq ChIP-seq kit, and the details can be found on http://ngsc.med.upenn.edu/. According to the protocol, 10 ng of ChIP DNA product was used to start the library construction.