samples were subject to crosslinking in 1% formaldehyde for 5 min followed by quenching with glycine for 5 min on ice. Cells were pelleted by centrifugation and resuspended in lysis buffer (50 mM HEPES pH 8.0, 140 mM NaCl, 1 mM EDTA, 10% glycerol, 0.5% NP40, 0.25% Triton X100). The pellets were then resuspended in washing buffer (10 mM Tris pH 8.0, 1 mM EDTA, 0.5 mM EGTA, 200 mM NaCl), washed and resuspended in shearing buffer (0.1% SDS, 1mM EDTA, pH 8, 10mM Tris HCl, pH 8) before sonication using Covaris E220 following manufacturer's instruction. Chromatin fragments were precipitated using specific antibodies and Protein G beads, washed and treated with Proteinase K and RNAse A. The ChIP complexes were washed and eluted in 50 mM NaHCO3 and 1% SDS, followed by overnight reverse crosslinking. The recovered DNAs from ChIP and ChIP-reChIP were used for library generation. Libraries were quantified with the Bioanalyzer 2100.