Carbonic anhydrase IX (CA IX) is a member of the CA family of zinc-binding enzymes that catalyze a reversible conversion between carbon dioxide and carbonic acid, in a reaction that involves facilitated hydration of CO2 to H2CO3 followed by the spontaneous dissociation of H2CO3 into bicarbonate and proton. The CA IX molecule consists of a large extracellular domain (ECD), single-pass transmembrane region (TM) and a short intracytoplasmic (IC) tail. The ECD contains an N-terminally located PG-like region (which is absent from the other carbonic anhydrase isoforms) and a centrally located, well conserved catalytic domain (CA). CA IX is a cell surface protein that is present in human tumors, but not in the corresponding normal tissues. Moreover, expression of CA IX correlates with poor prognosis in many tumor types. CA IX plays a role in two phenomena involved in development of tumor phenotype - control of cell adhesion and pH regulation. Tight association of CA IX with tumors is to a major part related to tumor hypoxia.