library_construction_protocol: Cryo-preserved cells were thawed and immediately resuspended in 1% formaldehyde (diluted in PBS from a 37% stock, with 10-15% methanol, F8775-500ML, Sigma-Aldrich, St. Louis, MO) and incubated at room temperature for 10 minutes without shaking. Glycine was added to 125 mM to quench formaldehyde and cells were centrifuged at 2,400g for 5 minutes. The cell pellet was washed in 1 mL of cold PBS and centrifuged again. The supernatant was discarded and the cells were stored at -80°C until further processing. Chromatin immunoprecipitation was performed as previously described 11. Cells were sonicated for 8 cycles of 30 seconds with 30 seconds of rest on an Epishear probe-in sonicator (Active Motif). An antibody against H3K27me3 (Active Motif, cat # 39155) was used for pulldown and input DNA was used as controls for each sample. NEBNext ChIP-Seq Library Prep Reagent Set for Illumina