Frozen-stored mammary tissues from L1 and p13 were broken into powder with mortar and pestle. Nuclei were extracted in Farnham lysis buffer (5mM PIPES pH8.0, 85mM KCl, 0.5% NP-40 and protease inhibitors). Mammary epithelial cells were isolated from fresh L1 and p13 mammary tissues. Antibodies against STAT5A (Santa Cruz, sc-1081), GR (Thermo Scientific, PA1-511A), MED1 (Bethyl laboratory, A300-793A), H3K27ac (Abcam, Ab4729), NFIB (Santa Cruz, sc-5567), ELF5 (Santa Cruz, sc-9645), RNA Pol II (Abcam, Ab5408) and H3K4me3 (Milllipore, 17-614) were used for ChIP. Libraries were prepared for sequencing using standard Illumina protocols.