CA9 and neoplasm: Two of the carbonic anhydrases, CA9 and CA12, are transmembrane isozymes that are highly expressed in some tumours, and may be implicated in acidification of the extracellular milieu surrounding cancer cells, thus creating a microenvironment conducive to tumour growth and spread (Pastorek et al, 1994; Ivanov et al, 1998; Türeci et al, 1998; Parkkila et al, 2000).