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Claudin-1 is a member of the claudin family of membrane proteins that form tight junction strands. These strands help control passage between neighbouring cells. Expressing claudin-1 in fibroblasts that lack normal tight junctions can reconstitute strand networks.^[ref]

Its physiological importance became especially clear in claudin-1-deficient mice. Their epidermal barrier failed and they lost water rapidly, despite retaining other recognizable skin structures. This showed that a multilayered epidermis still depends on functional tight junctions, rather than deriving all barrier function from the mere presence of many layers.^[ref]

Mikio Furuse investigated the molecular construction of tight junctions. In a 1998 study, introducing claudin-1 or claudin-2 into fibroblasts that normally lack tight junctions produced networks of junction-like strands at contacts between cells. Immunofluorescence located the introduced proteins, while freeze-fracture imaging revealed the strand architecture.^[ref]

The experiment demonstrated that either claudin could build a key structural feature in a simplified cellular setting. Recruitment of occludin further distinguished a strand-forming component from another associated junctional protein.