CA9 and neoplasm: Tumour cells can maintain their pHi through increased expression and activation of pH regulatory proteins, some of which are HIF1-dependent, such as monocarboxylate transporter 4 (MCT4), which exports lactate and H+ from tumour cells, or carbonic anhydrase IX (CAIX), an enzyme that accelerates the conversion of CO2 and H2O to HCO3− and H+ [18,27,28,29].