Carbonic anhydrase IX, or CAIX, is a transmembrane protein whose expression is restricted to few normal tissues.
Under hypoxic conditions, hypoxia inducible factor-1 alpha (HIF-1a) is stabilized and the transcription factor HIF-1 transactivates the CA9 gene, resulting in expression of the CAIX protein in the hypoxic cells.
CAIX is involved in both the extracellular acidification of hypoxic tumors, as well as the regulation of intracellular pH, making it an attractive target for therapies based on the manipulation of pH.