Frozen-stored mammary tissues from L1 or 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). Antibodies against H3K27ac (Abcam, Ab4729), MED1 (Bethyl laboratory, A300-793A), RNA Pol II (Abcam, Ab5408), GR (Thermo Scientific, PA1-511A) were used for ChIP. Libraries were prepared for sequencing using standard Illumina protocols.