CA9 and neoplasm: CAIX is a metalloenzyme that catalyses the reversible formation of HCO3− and H+ ions from H2O and CO2. CAIX expression is promoted by hypoxia inducible factors 1α (HIF-1α) in the hypoxic regions within the tumour mass13,14 and also by extracellular acidic microenvironment via HIF-1α-independent mechanisms15,.16