Estrogen receptor (ER) dimerization is prerequisite for its activation of target gene transcription.Because the two forms of ER, ER alpha and ER beta, exhibit opposing functions in cell proliferation, the ability of ligands to induce ER alpha/beta heterodimers vs.their respective homodimers is expected to have profound impacts on transcriptional outcomes and cellular growth.However, there is a lack of direct methods to monitor the formation of ER alpha/beta heterodimers in vivo and to distinguish the ability of estrogenic ligands to promote ER homo-vs.heterodimerization.Here, we describe bioluminescence resonance energy transfer (BRET) assays for monitoring the formation of ER alpha/beta heterodimers and their respective homodimers in live cells.We demonstrate that although both partners contribute to heterodimerization, ligand-bound ER alpha plays a dominant role.Furthermore, a bioactive component was found to induce ER beta/beta homodimers, and ER alpha/beta heterodimers but had minimal activity on ER alpha/alpha homodimers, posing a model that compounds promoting ER alpha/beta heterodimer formation might have therapeutic value.Thus, ER homodimer and heterodimer BRET assays are applicable to drug screening for dimer-selective ER modulators, which may potentially dampen the proliferative potential of ER alpha in breast cancer cells.