Channel-forming tight junction protein with selectivity for anions, including chloride and bicarbonate, and for solutes smaller than 9 Angstrom in diameter. In the kidney proximal tubule, may be involved in quantitative reabsorption of filtered anions. Does not affect water permeability. Interacts with OCLN. Expressed at high levels in the kidney and at mucher lower levels in the brain. In the kidney, expression gradually decreases from the proximal tubule downstream to the distal convoluted tubule. Expressed in the thin ascending limb of Henle's loop, as well as in the thick ascending limb of Henle's loop. In the distal convoluted tubules, expressed only in a few tubules. Not detected in the collecting duct. In the brain, expressed in blood vessels (at protein level). Belongs to the claudin family.