TFRC and breast cancer: As shown in Figure 3A, Gd-HFn NPs substantially bound to MDA-MB-231 human breast cancer cells and the binding was significantly blocked by adding an excess of anti-TfR1 monoclonal antibody (mAb) as determined by flow cytometric analysis, confirming a specific binding of Gd-HFn to TfR1 on MDA-MB-231 tumor cells.