Acquired resistance to endocrine therapy represents a major clinical obstacle to the successful management of estrogen-dependent breast cancers expressing estrogen receptor (ER). Because a switch from ligand-dependent to ligand-independent activation of ER-regulated breast cancer cell growth and survival may define a path to endocrine resistance, enhanced mechanistic insight concerning the ligand-independent fate and function of ER, including a more complete inventory of its ligand-independent cofactors, could identify novel markers of endocrine resistance and possible targets for therapeutic intervention in breast cancer. Here, we identify the deleted in breast cancer 1 gene product, DBC-1 (KIAA1967), to be a principal determinant of unliganded ER expression and survival function in human breast cancer cells. The DBC-1 amino terminus binds directly to the ER hormone-binding domain both in vitro and in vivo in a strict ligand-independent manner. Furthermore, like estrogen, the antiestrogens tamoxifen and ICI 182,780 disrupt the DBC-1/ER interaction, thus revealing the DBC-1/ER interface to be a heretofore-unrecognized target of endocrine compounds commonly used in hormonal therapy. Notably, RNAi-mediated DBC-1 depletion reduces the steady-state level of unliganded, but not liganded ER protein, suggesting that DBC-1 may stabilize unliganded ER by virtue of their direct association. Finally, DBC-1 depletion promotes hormone-independent apoptosis of ER-positive, but not ER-negative, breast cancer cells in a manner reversible by endocrine agents that disrupt the DBC-1/ER interaction. Collectively, these findings establish a principal biological function for DBC-1 in the modulation of ER expression and hormone-independent breast cancer cell survival.