Carbonic anhydrase 9 is an enzyme that in humans is encoded by the CA9 gene.
Carbonic anhydrases (CAs) are a large family of zinc metalloenzymes that catalyze the reversible hydration of carbon dioxide. They participate in a variety of biological processes, including respiration, calcification, acid-base balance, bone resorption, and the formation of aqueous humor, cerebrospinal fluid, saliva, and gastric acid. They show extensive diversity in tissue distribution and in their subcellular localization. CA IX is a transmembrane protein and the only tumor-associated carbonic anhydrase isoenzyme known. It is expressed in all clear-cell renal cell carcinoma, but is not detected in normal kidney or most other normal tissues. It may be involved in cell proliferation and transformation. This gene was mapped to 17q21.2 by fluorescence in situ hybridization, however, radiation hybrid mapping localized it to 9p13-p12.
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 biomarkers of hypoxic regions in many solid tumors.