Whilst, radioiodine ablation is an effective therapy for many patients with thyroid cancer, a subset of patients are incapable of accumulating sufficient iodide-131 for effective treatment, due to low sodium–iodide symporter (NIS) activity. Previous work has identified that the overexpression of pituitary tumor transforming gene (PTTG) binding factor (PBF) in thyroid cells leads to the redistribution of NIS from the plasma membrane into intracellular vesicles, thereby reducing radioiodine uptake. With radioiodine being proposed as a potential treatment for breast carcinomas, where PBF has been reported to be overexpressed, it is important to discern the relationship between PBF and NIS in breast cancer cell lines. Immunofluorescent microscopy revealed co-localisation between NIS and PBF in co-transfected MCF7 and T47D cells, with increased intracellular staining for NIS compared to cells transfected with NIS alone. We have recently identified PBF as a tyrosine phosphorylated protein, with Src phosphorylation at residue Y174 critical to NIS regulation in thyroid cells. Importantly, phosphorylated PBF co-localised at the plasma membrane with NIS in T47D breast cells. In functional studies using iodine-125 in MCF7 and MDA-MB-231 cells, PBF significantly repressed radioiodine uptake in cells expressing exogenous NIS (25% and 30% reduction respectively, both n=3 and P<0.05). Treatment with PP1, a Src-inhibitor shown to inhibit the phosphorylation of PBF, restored the ability of MCF7 and MDA-MB-231 cells to uptake iodine-125 (1.24- and 1.69-fold increase respectively, n=3, P<0.05). PBF transfection also repressed radioiodine uptake in MCF7 cells treated with the NIS-inducing reagents all-trans retinoic acid (ATRA) and dexamethasone (31% reduction, both n=3 and P<0.05), with preliminary data suggesting that this reduction can also be overcome using PP1 treatment. Taken together, these data suggest that PBF can alter the subcellular location of NIS and thereby reduce the ability of breast carcinoma cell-lines to take up iodide, consistent with those findings previously reported in thyroid cells.