ESR1 and breast cancer: To that end, we have synthesized two new CAs targeted to the ER, which are composed of Gadolinium chelate of pyridine-tetra-acetate (PTA-Gd) conjugated with the native ligand 17β-estradiol (EPTA-Gd) or with the antiestrogen tamoxifen (TPTA-Gd), and evaluated their MRI properties in solution, in breast cancer cells and in breast cancer xenografts in animal models (16–18).