CA9 and breast cancer: 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.