CA9 and cancer: Cancer cells maintain pHi partially through increased activation and expression of HIF1-dependent genes [6], such as SLC16A3 [monocarboxylate transporter 4 (MCT4)] which removes lactate along with hydrogen ions from tumour cells, or CA9 [carbonic anhydrase IX (CAIX)] which facilitates the formation of bicarbonate and hydrogen ions from CO2 and H2O [8, 9].