Cells were lysed by beads-beating and following sonication to fragment the chromatin to an average size around 100-400bp. The DNA were purified with ethanol precipitation. The purified DNA were further heat denatured for ssDNA library preparation. DNA prepared above were immobilized on MyOne C1 beads (Life Technology). Primers were added by PCR and adaptors by ligation. Then DNA were eluted as ssDNA at 95°C. The ssDNA were amplified by PCR reaction, and the amount of each sample was verified by qPCR. Verified sample DNA were sequenced by Illumina HiSeq generating 100bp pair-ended reads.