The carbonic anhydrases (or carbonate dehydratases) form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons (or vice versa), a reversible reaction that occurs rather slowly in the absence of a catalyst. CAIX is considered to be one of the best cellular biomarkers of hypoxia. Furthermore, recent studies examining the association between CAIX levels and various clinicopathological outcomes suggest that CAIX expression may also be a valuable prognostic indicator for overall survival. Antibodies against CAIX serve as excellent excellent biomarkers of hypoxic regions in many solid tumors.