Occludin is a 552 amino acid (aa), predicted molecular weight 65 kDa integral membrane protein that localizes within tight junctions of epithelial and endothelial cells . Human Occludin shares 90% aa sequence identity with the mouse ortholog. Occludin contains a cytoplasmic N-terminus (aa 1-66), a MARVEL domain, four transmembrane domains, and a long C-terminal cytoplasmic domain (aa 266-522). Within tight junctions, Occludin associates with ZO-1 and the transmembrane protein, Claudin to create an barrier that regulates the paracellular permeability of ions, small molecules, and macromolecules. Occludin plays important roles in establishing and maintaining the blood-brain barrier and the integrity of epithelia in various tissues. Post-translational modifications of Occludin, such as Ser490 phosphorylation and subsequent ubiquitination, modulate trafficking to the cell membrane and tight junction permeability. Changes in Occludin expression and localization are associated with epithelial to mesenchymal transition and metastasis.