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 (PubMed:20375010). In the kidney, expressed in the proximal tubule and in the Henle's loop. In the distal convoluted tubule, not expressed in all tubules. Not detected in the collecting duct (at protein level). Belongs to the claudin family.