Cells were chemically crosslinked by the addition of 4 mL of 1.5% formaldehyde in PBS per 10 cm2 dish for 15 min at room temperature. Cells were resusupended in Lysis Buffer 1 (50 mM HEPES , pH 7.5, 140 mM NaCl, 1 mM EDTA, 10% glycerol, 0.5% IGEPAL, 0.25% Triton X-100, 1X HALT protease inhibitor) and 3T3 mouse embryonic fibroblast cells (formaldehyde fixed as above) equivalent to 10% GM15850 cells were added to each sample for downstream ChIP-RX analysis. NGS libraries were generated with Thruplex DNAseq Single Index Kit (Rubicon) and sequenced (single end, 150 bp reads) via NextSeq 500.