Abstract Our laboratory has shown that thymocyte selection-associated HMG-box protein (TOX) is a member of a small subfamily of DNA binding factors, and a key regulator of multiple aspects of immune system development. Recently, genome wide association studies have mapped a breast cancer susceptibility locus to the gene encoding the TOX-family member TOX3. However, little is known concerning the pattern of expression or biological functions of TOX3 in breast cancer and normal mammary epithelial cells. We find that TOX3 is specifically expressed in estrogen receptor (ER+) mammary luminal epithelial cells. Interestingly, using a TOX3 monoclonal antibody developed by our laboratory, we identify a subset of breast tumors that highly express TOX3. Moreover, using the MCF-7 breast cancer cell line, we find that TOX3 regulates a subset of ER target genes in an estrogen independent but ER dependent manner, potentially revealing a novel mechanism of ER target gene activation. In addition, TOX3 enhances cell migration. Together, these studies lend support to the idea of altered TOX3 expression influencing breast cancer development and/or progression. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1311. doi:1538-7445.AM2012-1311