Frozen-stored mammary tissues 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), H3K4me3 (Millipore, 17-614), PolII (Abcam, ab5408) were used for ChIP. Libraries were prepared for sequencing using standard Illumina protocols.