Cells were harvested, resuspended and chemically cross-linked in suspension by the addition of 1% fresh formaldehyde with 9 minutes incubation at room temperature on a rotator. Cross-linking was quenched and cells were harvested by centrifugation and washed twice in cold PBS. Nuclei were extracted and sonication was performed using a Misonix S4000 sonicator. Fragment size between 200bp and 600bp was obtained and confirmed by gel electrophoresis and Bionalyzer. DNA library preparation and high-throughput sequencing were performed on an Illumina Genome Analyzer II following the protocol recommended by Illumina with a 75bp readleght at the Yale Center for Genome Analysis.