DNase-seq: L1 mammary gland was isolated and snap frozen in liquid nitrogen. Frozen tissue was grinded into powder and homogenized with buffer A (15 mM Tris–HCl pH 8.0, 15 mM NaCl, 60 mM KCl, 1 mM EDTA, 0.5 mM EGTA, 0.15 mM Spermine, 0.5 mM Spermidine, 0.5 mM DTT, 1 mM PMSF with proteinase inhibitors). After cells were lysed in buffer A supplemented with 0.2% NP40, nuclei were collected, counted and re-suspended in DNase buffer. 10 U DNase I (New England Biolab) was used to digest 10 million nuclei at 37◦C for 5 min followed by proteinase K digestion. Genomic DNA was then purified. Approximately, 50–100 bp DNA fragments were selected for further library construction and sequencing. Libraries were prepared for sequencing using standard Illumina protocols.