Estrogen signaling pathway is critical for breast cancer development and has remained the major adjuvant therapeutic target for this disease. Tamoxifen has been used in clinic for many years to treat ER-positive breast cancer. However a great many (30%) suffer relapse due to drug resistance. In this study, the bromodomain inhibitor JQ1 was found to down-regulate ERalpha gene expression and have anti-tumor effect in cultured tamoxifen-resisant breast cancer cells. We used microarrays to detail the global programme of gene expression in tamoxifen-resistant MCF7 cells treated with the bromodomain inhibitor JQ1. Overall design: Tamoxifen-resistant breast cancer MCF7 cells were treated with DMSO (vehicle) or JQ1 (0.2 uM) for 24 hours before total RNA was purified for microarray. Each sample was triplicated.