Abstract : Tamoxifen, a SERM (Selective Estrogen Receptor Modulator), is the most commonly used endocrine treatment for all stages of breast cancer. However, progression from tamoxifen sensitivity to tamoxifen resistance occurs in a substantial portion of the tumors. Full antiestrogens, such as ICI 182,7&0, are currently used as the second line therapy after-failure of long-term tamoxifen therapy. To facilitate the design and characterization of more appropriate therapeutic agents for endocrine therapy of breast cancer, it is very important to understand the functional mechanisms that distinguish full antiestrogens from SERMs. It has been shown that estrogen receptor (ER) can recruit corepressors NCoR (nuclear receptor corepressor) and SMRT (silencing mediator of retinoid and thyroid receptors) in the presence of tamoxifen, suggesting a possible role of N-CoR/SMRT in mediating the antagonist activity of tamoxifen. However, it is not clear if apo-ER or ICI 182,780-bound ER can recruit NCoR/SMRT or other corepressors. To investigate the possible involvement of different corepressors in the actions of different antiestrogens and unliganded ER, we have constructed a focused phage display library which contains the "CoRNR box" motif, a binding site important for N-CoR/SMRT to interact with the nuclear receptors. In this report, we have shown that screening of the CoRNR box library with ER treated with no hormone or different antiestrogens led to the isolation of peptides that differentially interact with apo-ER, tamoxifen bound ER, or ICI 182,780-bound ER. These interactions observed in vitro have also been confirmed in vivo using a mammalian two-hybrid assay. Using a series of ER mutants, we were able to show that these CoRNR box-containing peptides have different binding characteristics from the peptides that contain the coactivator LXXLL motif.