Recent pre-clinical studies have provided evidence that the androgen receptor (AR) can exert tumorigenic effects in estrogen receptor alpha (ERα) negative subtypes of breast cancer and have spawned clinical trials that employ anti-androgenic agents to treat women with advanced forms of this type of disease. The MDA-MB-453 cell line has been the major working model of AR-positive, ERα-negative breast cancer. This breast cancer cell line is growth stimulated by the native AR agonist, 5α-dihydrotestosterone (DHT) and growth inhibited by AR antagonists or AR ablation. However, many mechanistic conundrums remain, including the relative importance of a mutation in the ligand binding domain of the AR gene in MDA-MB-453 cells that alters transcriptional activity of the mutant receptor compared to the wild type receptor. In addition, medroxyprogesterone acetate (MPA), a synthetic progestin with potent AR agonist activity binds to the AR in MDA-MB-453 cells with an affinity and dissociation rate tantamount to DHT, but instead of promoting growth, MPA exerts a potent AR-dependent anti-proliferative effect. Therefore, even within this model cell line, the specific conditions required for AR to activate a tumorigenic transcriptional program are not clear. Furthermore, MFM223 cells, a less studied model of AR positive ERα-negative breast cancer that possesses a wild type AR is growth inhibited by treatment with DHT and MPA and growth stimulated by MDV3100, a new generation AR antagonist. We are currently undertaking experiments that employ ChIP-seq, RNA-seq, rapid immunoprecipitation mass spectrometry of endogenous proteins (RIME) and ex vivo culture of ERα-negative primary breast cancers to elucidate the parameters that dictate differential AR signalling events in ERα-negative breast cancer, with view to facilitating patient selection for AR targeting adjuvant therapies.