Chromatin Immunoprecipitation was performed as described (Zwart W et al. EMBO J. 2011, 30:4764-76) : 10ug of antibody was used, with 100ml of Protein A magnetic beads. Antibody used was raised to detect Estrogen Receptor alpha (HC-20, SC-543; Santa Cruz). DNA was amplified as described (Ross-Innes CS et al. Nature 2012;481(7381):389-93). Libraries were prepared according to Illumina's instructions accompanying the DNA Sample Kit (Part# 0801-0303).