10 μg of DNA was used to incubate with anti-Flag antibody (Sigma M2) or with same amount of mouse IgG as the negative control. BSA coated protein A agarous beads were used to obtain the ChIP product. Chromatin was reverse crosslinked at 65°C ~16hours, and DNA was purified by Qiagen PCR purification kit. After wash steps, cells were resuspended in lysis buffer and proceeded to the sonication with Diagenode Bioruptor (High power, 30 second cycles, 7.5 min total length X 3). Cell debris was removed by 10 min centrifuge, 15,000 rpm at 4°C. Extracted chromatin supernatant was used for the following chromatin immunoprecipitation or frozen in -80°C for storage. To generate the library from ChIP experiements, 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/. 10 ng of ChIP DNA product was used to start the library construction.