ChIP experiments were conducted as described previously (16 (link)) and were done in duplicates. MCF7, T47D, MDAMB134 and SUM44 cells were cultured for three days in HD conditions then treated with 10nM estradiol for 45 minutes. For the ChIP experiments with tamoxifen we used 10nM 4-hydroxytamoxifen for 45 minutes. Chromatin from 20 million formaldehyde-fixed cells was sonicated to a size range of 200–300 bp. Solubilized chromatin was immunoprecipitated with a mix of the ER antibodies Ab10 (Thermo Fisher Scientific) and SC-543 (Santa Cruz), a mix of the FOXA1 antibodies ab5089 and ab23738 (Abcam), GATA3 antibody D13C9 (Cell signaling) or H3K27Ac antibody (C15410196, Diagenode). The same antibodies were used in each experiment for all the cell lines. The samples were reversed crosslinked, treated with proteinase K, and DNA was extracted. Libraries were sequenced using 75 bp paired-end reads on the Illumina Nextseq500 at the Dana-Farber Cancer Institute.