Carbonic anhydrases IX (CAIX), also known as membrane antigen MN or CA9, is a member of the carbonic anhydrase (CA) family and may be involved in cell proliferation and cellular transformation. CAs are zinc metalloenzymes that catalyze the reversible hydration of carbon dioxide ( H2O + CO2 = H+ + HCO3- ) and thus participate in a variety of biological and physical processes. CAIX is a transmembrane protein structurally consisting of a signal peptide, a proteoglycan-related region, a CA domain with a highly conserved active site, a transmembrane anchor and an intracytoplasmic tail, and is the only tumor-associated CA isoenzyme known so far. Compared with normal tissues, CAIX is overexpressed in a wide spectrum of tumor types and associated with increased metastasis and poor prognosis in aggressive carcinomas. CAIX expression is cell density dependent and has been shown to be strongly induced by hypoxia, accordingly facilitates adaptation of tumor cells to hypoxic conditions. CA9 is regarded as a new therapeutic target for CA9-derived carcinomas.