Occludin is a novel integral membrane protein localized at tight junctions (TJs). Researchers identified this protein by isolating a junctional fraction from chick liver and using monoclonal antibodies (mAbs) to detect a 65-kD membrane protein. Immunofluorescence and immunoelectron microscopy confirmed that this protein is exclusively localized at TJs in both epithelial and endothelial cells. The protein was found to be an integral membrane protein, as it could not be extracted from plasma membranes without detergent. The cDNA encoding occludin was cloned and sequenced, revealing no homology to other known proteins. The predicted amino acid sequence showed four hydrophobic, potentially membrane-spanning domains, similar to connexin, an integral membrane protein in gap junctions. These findings confirmed that occludin is an integral membrane protein localized at TJs. Occludin is designated as the protein responsible for TJ localization, and its identification provides a molecular basis for understanding TJ structure and function. The study also highlights the importance of TJ in creating a barrier to solute diffusion and establishing cell polarity. Occludin's role in TJ formation is distinct from ZO-1, which is localized at the undercoat of TJs. The identification of occludin opens new avenues for studying TJ at the molecular level.Occludin is a novel integral membrane protein localized at tight junctions (TJs). Researchers identified this protein by isolating a junctional fraction from chick liver and using monoclonal antibodies (mAbs) to detect a 65-kD membrane protein. Immunofluorescence and immunoelectron microscopy confirmed that this protein is exclusively localized at TJs in both epithelial and endothelial cells. The protein was found to be an integral membrane protein, as it could not be extracted from plasma membranes without detergent. The cDNA encoding occludin was cloned and sequenced, revealing no homology to other known proteins. The predicted amino acid sequence showed four hydrophobic, potentially membrane-spanning domains, similar to connexin, an integral membrane protein in gap junctions. These findings confirmed that occludin is an integral membrane protein localized at TJs. Occludin is designated as the protein responsible for TJ localization, and its identification provides a molecular basis for understanding TJ structure and function. The study also highlights the importance of TJ in creating a barrier to solute diffusion and establishing cell polarity. Occludin's role in TJ formation is distinct from ZO-1, which is localized at the undercoat of TJs. The identification of occludin opens new avenues for studying TJ at the molecular level.