The claudin superfamily consists of structurally related proteins that are important structural and functional components of tight junctions. Claudin-23, also known as CLDN23 or CLDNL, is a 292 amino acid multi-pass membrane protein that localizes to cell junctions and belongs to the claudin family. Expressed in stomach and placenta, as well as in germinal center B-cells, claudin-23 is thought to exhibit calcium-dependent cell-adhesion activity through which it plays an essential role in tight junction-specific obliteration of the intercellular space. Human claudin-23 shares 80% sequence similarity with its mouse counterpart, suggesting a conserved role between species. Overexpression of claudin-23 is associated with colon tumors, implicating claudin-23 as a possible metastasis factor.