Cells were crosslinked with 1% formaldehyde for 10 min and neutralized with 0.125 M glycine for 5 min. The lysates were sonicated on ice using a SCIENTZ-IID ultrasonic homogenizer A 10 µl aliquot of the sonicated lysate was used for analyzing DNA size distribution. This aliquot was incubated with 110 µl of water containing 0.2 M NaCl at 65°C for 3-6 h to reverse crosslinking, followed by treatment with 0.2 mg/ml RNase A (Transgenic Biotechnology #GE101-01) for 1 h at 37°C, then with 0.5 mg/ml proteinase K (Transgenic Biotechnology #GE201-01) for 1 h at 55°C. DNA was purified using a HiPure Gel DNA Mini Kit (Magen #D2111-03). DNA size distribution was analyzed by agarose gel electrophoresis. This DNA sample also served as an input control. After an optimal DNA fragment size range of 100-500 bp was achieved, immunoprecipitation reaction was carried out by incubating 40 µl of FLAG affinity gel (Merck Millipore #F2426) and 280 µl of the sonicated cell lysate in 1 ml of ChIP dilution buffer [20 mM Tris-HCl (pH 8), 150 mM NaCl, 2 mM EDTA (pH 8), 1% Nonidet P-40, 0.1% sodium deoxycholate, and 0.01% SDS], supplemented with protease inhibitors (1 tablet per 50 ml, Roche #K562) with rotation for 3 h at 4°C.