For ChIP-seq, chromatin was crosslinked with 1 % formaldehyde (Sigma) by the addition of 1 ml 10x crosslinking buffer (50 mM HEPES, 1 mM EDTA, 0.5 mM EGTA, 100 mM NaCl, 10 % formaldehyde) to 10E7 cells in 9 ml of media and incubation at room temperature for 10 minutes. Crosslinking was quenched with 130 mM glycine, and cells were washed with cold PBS before snap freezing pelleted cells. Fixed material was stored at -80 ºC for less than 12 months. Chromatin immunoprecipitation was performed using Agarose ChIP Assay Kit (Merck Millipore). Briefly, 10E7 cells were lysed by incubation on ice with 130 µl lysis buffer for 15 minutes. Lysed cells were transferred to Covaris microtubes and sonicated on the Covaris S220 (Duty cycle: 2 %, Intensity: 3, Cycles per burst: 200, Power mode: Frequency sweeping, Duration: 480 sec, Temp.: 6 ºC) to generate 200-400 bp fragments. Insoluble material was removed by centrifugation (15,000 rcf, 15 min, 4 ºC) and soluble material was diluted to 4 ml with dilution buffer. Immunoprecipitation was performed by incubation of 2 ml diluted chromatin (equivalent to 5x10E6 input cells) with antibodies for Gata-1 (7 µg ab11852, lot: GR290167-7; AbCam), H3K4me3 (1 µl 07-473, lot: 2664283; Millipore), H3K27ac (0.3 µg ab4729, lot: GR3205523-1; AbCam), or CTCF (10 µl 07-729, lot: 2836929; Millipore) overnight. Chromatin binding to Protein A/agarose slurry, washes and elution were performed according to the manufacturer's instructions. DNA was purified by phenol-chloroform extraction with PhaseLock tubes (5Prime) and ethanol precipitation with NaOAc, and 2 µl GlycoBlue (Invitrogen). ChIP enrichment was determined by RT-qPCR NEBNext Ultra II DNA Library Prep kit for Illumina (New England Biolabs).