MEFs were fixed in DMEM containing 1% fresh formaldehyde and incubated at room temperature for 10 min, quenched with 250mM glycine for 10 min, and washed twice with ice cold PBS, before being flash-frozen at -80°C. Cross-linked cells were lysed and sonicated on a Bioruptor Plus (Diagenode) sonicator to fragment chromatin to an average length of 300bp. Immunoprecipitated DNA or 50 ng of input DNA was used for library preparation using the ThruPLEX DNA-Seq library preparation protocol (Rubicon Genomics, UK).