MCF-7 cells [American Type Culture Collection (ATCC)] were grown in DMEM with 2mM L-glutamine, 10% fetal bovine serum, 50U/ml penicillin and 50ug/ml streptomycin in 37C incubator with 5% CO2. LY2 cells (kind gift from R. Clarke, Georgetown, Washington D.C.) were grown in phenol red-free MEM with 2mM L-glutamine, 10% charcoal dextrin stripped fetal bovine serum, 50U/ml penicillin and 50ug/ml streptomycin in 37C incubator with 5% CO2. Cells were steroid depleted for 3 days prior to treatment with Tamoxifen for 45 minutes. Proteins were cross linked to DNA by treating with formaldehyde (1%) for 10 minutes (ER) or 60 minutes (HMGB2). Chromatin immunoprecipitation and preparation of libraries was carried out as described in Schmidt et al (2009) Methods 48:240-248.