The downstream targets of HIF-1α, carbonic anhydrase IX (CAIX), glucose transporter 1 (GLUT1) and C-X-C chemokine receptor type 4 (CXCR4) [17-20], and insulin-like growth factor 1 receptor (IGF1R) that maintains the hypoxia response via HIF-1α stabilization [21-23], are expressed on the plasma membrane of breast cancer cells and are therefore potentially suitable candidates for molecular imaging of hypoxic tumors with antibodies or antibody fragments. Here, CXCR4 is linked to breast cancer.