As reported by Hu et al. [54], carbonic anhydrase IX (CA-IX) catalyzes the carbon dioxide (CO2), produced by the mitochondrial respiration in tumor cells, into bicarbonate ions (HCO3−) and protons (H+), and then extrudes H+ into the extracellular environment, leading to acidification of the microenvironment. Here, CA9 is linked to neoplasm.