Cells were fixed in culture media containing 1% formaldehyde (Sigma) for 10 minutes at room temperature. Following fixation, cells were resuspended in [10 mM HEPES pH 7.5,10 mM EDTA, 0.5 mM EGTA, 0.75% Triton X-100, Complete protease inhibitors] and incubated mixing end over end for 10 minutes at 4oC. Nuclei were pelleted by centrifugation, then resuspended and incubated as before in [10 mM HEPES pH 7.5, 200 mM NaCl, 1 mM EDTA, 0.5 mM EGTA, Complete protease inhibtors]. Subsequently nuclei were resuspended in sonication buffer [150 mM NaCl, 25 mM Tris pH 7.5, 5 mM EDTA, 1% Triton X-100, 0.1% SDS, 0.5% Sodium Deoxycholate, Complete protease inhibitors (Roche)], incubated for 30 minutes on ice, and sonicated in a Bioruptor Pico (Diagenode) for 10 cycles (30 seconds on, 30 seconds off). Insoluble chromatin was pelleted by high-speed centrifugation and the supernatant was incubated with the ChIP antibody over night mixing end over end at 4oC. 1μg antibody was used per million sonicated ES cells. Following over night incubation, antibody/chromatin complexes were pulled down by mixing 3 hours at 4oC with protein G Dynabeads (Thermo Fihser Scientific) using 5μl beads per μg antibody. Beads were washed 3 times and the DNA was eluted by shaking the beads over night at 65oC in [1xTE, 1% SDS, 0.5mg/ml proteinase K (Sigma)]. Eluted DNA was isolated by phenol/chloroform extraction and ethanol precipitation. Purified DNA was quantified using the Quant-iT PicoGreen kit (Thermo Fisher Scientific). Libraries were prepared from 10ng ChIP DNA using the NEBNext Ultra II DNA Library Prep kit for Illumina (E7645, New England Biolabs).