Plays a major role in tight junction-specific obliteration of the intercellular space, through calcium-independent cell-adhesion activity. Localizes to tight junctions in epithelial cells. Isoform A1: Expression is restricted to the lung (PubMed:19047087). Isoform A2: Expression is restricted to the stomach mucosa where it is predominantly observed in the epithelial cells of the pit region and the base of the gastric glands including exocrine and endocrine cells (at protein level) (PubMed:19047087). Belongs to the claudin family.