Frozen-stored mammary tissues from 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 inhibitor). Antibodies against EZH1 (gift from Sartorelli, Mousavi et al. Mol. Cell 2012), EZH2 (Diagenode, cat# C15410039 and lot# 003), STAT5 (Santa Cruz Biotechnology, cat# sc-1081; L-20, lot# L2012, 10ug) H3K27me3 (Millipore, 07-449) and PolII (Abcam, cat# ab5408, lot# GR106949-6) were used for ChIP. Libraries were prepared for sequencing using standard Illumina protocols.