ChIP-seq: Frozen-stored mammary tissues from p13 were broken into powder with mortar and pestle. With MECs, cells were cross-linked with 1% formaldehyde for 10 min at RT. Nuclei were extracted in Farnham lysis buffer (5mM PIPES PH8.0, 85mM KCl, 0.5% NP-40 and protease inhibitor). Antibodies against STAT5 (Santa Cruz Biotechnology, cat# sc-1081; L-20, lot# L2012, 10ug) and H3K4me3 (Millipore, cat# 17-164, lot# JBC1793805, 5ug) were used for ChIP. Active Motif performed ChIP with in-house H3K27me3 antibody using pooled tissues from three individual mice of p13. Libraries were prepared for sequencing using standard Illumina protocols.