Occludin is a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 that in humans is encoded by the OCLN gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

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Occludin is a 65-kDa (522-amino acid polypeptide -human) integral plasma-membrane protein located at the tight junction

Tight junction

Tight junctions, or zonula occludens, are the closely associated areas of two cells whose membranes join together forming a virtually impermeable barrier to fluid. It is a type of junctional complex present only in vertebrates...

s, described for the first time in 1993 by Shoichiro Tsukita. Together with the Claudin

Claudin

Claudins are a family of proteins that are the most important components of the tight junctions, where they establish the paracellular barrier that controls the flow of molecules in the intercellular space between the cells of an epithelium...

 group of proteins, it is the main component of the tight junctions.
According to its overall hydrophilicity, occludin appears to span the plasma membrane four times, forming two extracellular loops and exposing its NH2 and COOH terminus to the cytosol.