CLDN1 (SEMP1) is a membrane protein of the claudin family. It is a component of tight junction complexes that regulate the permeability of epithelia. It is required to prevent the paracellular diffusion of small molecules through these junctions in the epidermis and for the normal barrier function of the skin. It is necessary for normal water homeostasis and to prevent excessive water loss through the skin, likely via an indirect effect on the expression levels of other proteins, since CLDN1 itself seems to be dispensable for water barrier formation in keratinocyte junctions. Additionally, CLDN1 acts as a coreceptor for HCV entry into hepatic cells. In cancer, CLDN1 is a useful target in immunohistochemistry for identifying hemangiopericytoma and meningiomas, perineuriomas, ovarian adenocarcinomas in effusions, and as a general marker for epithelial differentiation. In normal tissue, CLDN1 shows particulate membranous staining in squamous epithelium and positivity in gastrointestinal, immune, endocrine, exocrine, muscle, lung, renal, kidney, breast, skin and reproductive tissues. References: J Invest Dermatol. 2013 May;133(5):1161-9, PMID: 23407391; Am J Surg Pathol 2002;26:1620, PMID:12459629; Mod Pathol 2005;18:1403, PMID:15920547; Am J Clin Pathol 2007;127:928, PMID:17509990; https://www.ncbi.nlm.nih.gov/pubmed/18520438, Am J Surg Pathol 2008;32:1088, PMID:18520438; Am J Clin Pathol 2006;125:203, PMID:16393681;