Claudin-1 (CLDN-1) is a member of the claudin integral membrane tight junction protein family.  Claudins are responsible for regulating paracellular permeability, promoting cell adhesion and maintaining cell polarity and barrier function.  Localized to the cytosol and membrane, Claudin-1 is broadly expressed in most tissues and is localized to most epithelial cells.  Surrounding secretory epithelial cells of stroma components such as fibroblasts, pneumocytes and lymphocytes do not express Claudin-1.  Claudin-1 expression is typically reduced in proliferating compared to quiescent epithelial cells.  Claudin-1 was immuno-negative in 98% of prostate cancer cases and significantly decreased in colon carcinoma tissue compared to adjacent normal mucosa.  Furthermore, Claudin-1 has demonstrated utility for identifying squamous cell carcinomas (SqCC).  In lung tumors, Claudin-1 expression was detected in 78% of SqCC but only 17% of adenocarcinoma.  Its upregulation is observed in pancreatic, thyroid, lung and oral SqCC.