Carbonic anhydrase IX (CAIX) is a member of the Carbonic anhydrase family which are enzymes that assists rapid interconversion of carbon dioxide and water into carbonic acid, protons, and bicarbonate ions. They are abundant in all mammalian tissues and because of their functionality, they have become an important diagnostic marker for various cancers, most notably renal cell carcinoma (RCC). Carbonic Anhydrase IX (CA IX) is one of the most hypoxically inducible genes because of its stability and the location of the expressed protein within the membrane. Carbonic anhydrases have a widespread role in regulating pH in normal tissues, by regulating hydrogen ion (H+) flux. The pH is important in cell death under hypoxia, thus a blockade of CA IX (CA9) results in increased cell death under hypoxia. Therefore, CA IX has become a reliable histochemical marker of hypoxia.