Cancer tissues have a hypoxic environment caused by limited blood supply; under these conditions, cancer cells elevate their production of carbonic anhydrase IX, an enzyme that catalyzes the reversible conversion of carbon dioxide to bicarbonate and a proton, referring to its contribution to an acidic microenvironment, so the extracellular pH (pHe) of cancer cells is lower (pH 6.2–6.9) than that of normal cells (pH 7.3–7.4) (68). Here, CA9 is linked to cancer.