Carbonic anhydrase 4 is an enzyme that in humans is encoded by the CA4 gene. It is mapped to 11; 11 C. Carbonic anhydrases (CAs) are a large family of zinc metalloenzymes that catalyze the reversible hydration of carbon dioxide. They participate in a variety of biological processes,including respiration,calcification,acid-base balance,bone resorption,and the formation of aqueous humor,cerebrospinal fluid,saliva,and gastric acid. They show extensive diversity in tissue distribution and in their subcellular localization. This gene encodes a glycosylphosphatidyl-inositol-anchored membrane isozyme expressed on the luminal surfaces of pulmonary (and certain other) capillaries and proximal renal tubules. Its exact function is not known; however,it may have a role in inherited renal abnormalities of bicarbonate transport.