Estrogen receptors (ERs), which mediate estrogen actions, regulate cell growth and differentiation of a variety of normal tissues and hormone-responsive tumors through interaction with cellular factors. In this study, we show that thyroid transcription factor-2 (TTF-2) is expressed in mammary gland and acts as ER alpha co-repressor. TTF-2 inhibited ER alpha transactivation in a dose-dependent manner in MCF-7 breast cancer cells. In addition, TTF-2 directly bound to and formed a complex with ER alpha, colocalizing with ER alpha in the nucleus. In MCF-7/TTF-2 stable cell lines, TTF-2 repressed the expression of endogenous ER alpha target genes such as pS2 and cyclin D1 by interrupting ER alpha binding to target promoters and also significantly decreased cell proliferation. Taken together, these data suggest that TTF-2 may modulate the function of ER alpha as a corepressor and play a role in ER-dependent proliferation of mammary cells. (C) 2012 Elsevier Inc. All rights reserved.