CA9 and neoplasm: Hypoxic tumor cells regulate the internal pH through the modulation of cell surface molecules such as extracellular carbonic anhydrases (CAIX and CAXII) and monocarboxylate transporters (especially MCT4), as proton export and lactate extrusion mechanisms responsible for extracellular acidification, which drives malignant progression [8,9], increases cell survival and invasiveness, and contributes to modifying immune cell function [10].