Estrogen Receptor (ER) drives proliferation in breast cancers and drugs such as tamoxifen and Aromatase Inhibitors, that target ER activity, are first line treatments in clinical practice. However drug resistance is a significant clinical problem. My laboratory has reported that chromatin-modifying pioneer factors are required for ER to bind the genome, and may constitute a unique opportunity for treating drug resistant cancer. My proposal consists of two complementary approaches to comprehensively explore how Estrogen Receptor interacts with these factors to direct transcription. (1) We will demonstrate that FoxA1 and the Groucho protein TLE1 are critical mediators of ER-chromatin interactions by mapping TLE1 binding sites on a genome-wide basis, and functionally testing the roles these factors play with ER in genomic remodeling, gene transcription, cell proliferation, and endocrine resistance. (2) More globally, to characterise ER transcriptional partners on a molecular basis, we will identify the complete complement of ER-associated proteins using novel proteomic approaches. Taken together, these approaches will explore how ER employs pioneer factors mechanistically, and will identify other potential players.