Plays a major role in tight junction-specific obliteration of the intercellular space, through calcium-independent cell-adhesion activity. Can form homo- and heteropolymers with other CLDN. Homopolymers interact with CLDN1 and CLDN2 homopolymers. Directly interacts with TJP1/ZO-1, TJP2/ZO-2 and TJP3/ZO-3. (Microbial infection) Interacts with Clostridium perfringens enterotoxin CPE; the interaction may disrupt claudin assembly in tight junctions. Expressed at high levels in liver and lung and, at lower levels, in kidney and testis. Belongs to the claudin family.