CXCR4 and breast cancer: In the past years a number of molecular targets for receptor targeted nuclear imaging of BC have been identified and are currently under investigation: hormone receptors, HER2, the somatostatin receptor (SSTR), the gastrin releasing peptide receptor (GRPR), folate receptor (FR), C-X-C chemokine receptor type 4 (CXCR4), neuropeptide Y receptor Y1 (NPY1R), and vasoactive intestinal polypeptide receptor 1 (VIP-R1).