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A ubiquitous membrane transport protein found in the plasma membrane of diverse cell types and tissues, and in nuclear, mitochondrial, and Golgi membranes. It is the major integral transmembrane protein of the erythrocyte plasma membrane, comprising 25% of the total membrane protein. It exists as a dimer and performs the important function of allowing the efficient transport of bicarbonate across erythrocyte cell membranes in exchange for chloride ion.