CA9 and neoplasm: CA IX is a highly active enzyme belonging to the family of carbonic anhydrases that catalyse the inter-conversion between carbon dioxide and bicarbonate plus proton and are vital for ion transport and acid-base balance in virtually all organisms.13 Out of 12 catalytically active human isoforms, CA IX is the only isoenzyme that is predominantly associated with tumours and rarely expressed in differentiated healthy tissues.