Cells were crosslinked with formaldehyde treatment and chromatin fragmented to 200 to 300 bp by sonication. Chromatin from 2 × 107 cells was used for each ChIP experiment, which yielded approximately 200 ng of DNA. Lysates were clarified from sonicated nuclei and histone-DNA complexes were isolated with antibody. Libraries were prepared according to Illumina's instructions. The ChIP DNA ends were repaired using PNK and Klenow enzyme, followed by treatment with Taq polymerase to generate a protruding 3′ A base used for adaptor ligation. Following ligation of a pair of Solexa adaptors to the repaired ends, the ChIP DNA was amplified using the adaptor primers for 17 cycles and the fragments around 220 bp (mononucleosome + adaptors) isolated from agarose gel. Libraries were sequenced on the Hi-seq following the manufacturer's protocols. As described in Barski et al., 2007 (A. Barski, S. Cuddapah, K. Cui, T.Y. Roh, D.E. Schones, Z. Wang, G. Wei, I. Chepelev and K. Zhao, High-resolution profiling of histone methylations in the human genome, Cell 129 (2007), pp. 823-837).