Claudins function as major constituents of the tight junction complexes that regulate the permeability of epithelia. While some claudin family members play essential roles in the formation of impermeable barriers, others mediate the permeability to ions and small molecules. Often, several claudin family members are coexpressed and interact with each other, and this determines the overall permeability. 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 (PubMed:23407391). (Microbial infection) Acts as a co-receptor for hepatitis C virus (HCV) in hepatocytes (PubMed:17325668, PubMed:20375010, PubMed:24038151). Associates with CD81 and the CLDN1-CD81 receptor complex is essential for HCV entry into host cell (PubMed:20375010). Acts as a receptor for dengue virus (PubMed:24074594). Homopolymers interact with CLDN3, but not CLDN2, homopolymers. Can form homo- and heteropolymers with other claudin family members (PubMed:23704991). Directly interacts with TJP1/ZO-1, TJP2/ZO-2 and TJP3/ZO-3. Interacts with MPDZ and PATJ (By similarity). Interacts with OCLN, CLDN4, CLDN6 and CLDN9 (PubMed:20375010). Interacts with CD81 (PubMed:20375010, PubMed:21516087). (Microbial infection) Interacts with hepatitis c virus heterodimer E1/E2. (Microbial infection) Interacts with dengue virus small envelope protein M. Associates with CD81 and the CLDN1-CD81 complex localizes to the basolateral cell membrane. Strongly expressed in liver and kidney. Expressed in heart, brain, spleen, lung and testis. The disease is caused by variants affecting the gene represented in this entry. Belongs to the claudin family.