CA9 and cancer: Cancer cells primarily express the plasma-membrane-associated CA isoforms CAIX and CAXII, as well as intracellular CAs such as CAI and CAII.96–104 Amongst the cancer-related CAs, CAIX has gained most attention, since expression of this isoform in healthy tissue is restricted to epithelial cells in the stomach and gut, but is strongly upregulated in many tumour tissues.105–107 The CAIX protein comprises an extracellular-facing catalytic domain tethered to the plasma membrane with a single transmembrane domain, and a short intracellular C-terminal tail.