Carbonic anhydrase IX (CA9/CAIX) is an enzyme encoded by the CA9 gene. It is one of the 14 subtypes of carbonic anhydrase discovered by humans, a transmembrane dimer metalloenzyme with extracellular active sites that promote gastrointestinal acid secretion. CAIX is overexpressed in many types of cancers, including clear cell renal cell carcinoma (RCC), cervical cancer, breast cancer, and lung cancer. In these cancers, CAIX promotes tumor growth by enhancing tumor acidosis.