CXCR4 and breast cancer: As a response, novel targeted radiotracers are being developed that are directed towards other biomarkers, such as the fibroblast activation protein (FAP) expressed on cancer-associated fibroblasts in the tumour stroma of many epithelial cancers [14, 15], the gastrin-releasing peptide receptor (GRPR) overexpressed in breast cancer (BC) and PCa [16–20] and C-X-C chemokine receptor type 4 (CXCR4) overexpressed in multiple cancers including, pancreatic cancer (PC), BC and PCa [19].