Na+/I- symporter (NIS)-mediated iodide uptake allows radioiodine therapy for thyroid cancer. NIS is also expressed in breast tumors, raising potential for radionuclide therapy of breast cancer. However, NIS expression in most breast cancers is low and may not be sufficient for radionuclide therapy. A better understanding of the mechanisms of NIS regulation in breast cancer may lead to strategies for increasing cell surface NIS and radioactive iodide uptake (RAIU) in breast cancer. The MCF-7 cell line is the only human breast cancer cell line with inducible endogenous NIS expression. Kogai et al.  first reported that trans-retinoic acid (tRA) induces NIS mRNA expression in MCF-7 cells and it was later reported that a combination treatment of tRA and hydrocortisone (tRA/H) further increases tRA-induced NIS expression/function in MCF-7 cells (Kogai et al., 2005; Dohan et al., 2006). In this study, we used gene expression profiling to identify genes that correlate with NIS expression in MCF-7 cells such that mechanisms underlying NIS modulation may be elucidated.