Carbonic anhydrase IX (CA9/CAIX) is one of the 14 carbonic anhydrase isoforms found in humans and is a transmembrane dimeric metalloenzyme with an extracellular active site that facilitates acid secretion in the gastrointestinal tract. CAIX is overexpressed in many types of cancer including clear cell renal cell carcinoma (RCC) as well as carcinomas of the cervix, breast and lung where it promotes tumor growth by enhancing tumor acidosis.