Claudin 1-8 proteins are a family of transmembrane proteins associated with tight junctions. Tight junctions are specialized regions of cell to cell contact, made up of a network of strands to act as a molecular gasket for preventing the leakage of ions, water etc., between cells. They are abundant in luminal epithelial sheets where they maintain epithelial cell polarity. Different tissues exhibit different claudin composition.

Claudin-1 is a protein that is encoded by the CLDN1 gene. CLDN1 is required to prevent the paracellular diffusion of small molecules through tight junctions in the epidermis and is required for the normal barrier function of the skin. Required for normal water homeostasis and to prevent excessive water loss through the skin, probably via an indirect effect on the expression levels of other proteins, since CLDN1 itself seems to be dispensable for water barrier formation in keratinocyte tight junctions

As Claudin-1 expression is low or undetectable in a number of breast tumors and breast cancer cell lines, it may be implicated as a possible tumor-suppressor gene. Claudin 1 has been shown to distinguish epithelial neoplasms from lymphomas, making it a useful marker for nearly all carcinomas.