We have raised an mAb to a previously undescribed 135-kD externally disposed integral membrane protein that is enriched in the intercellular junctional domain of cultured human umbilical vein endothelial cells. This protein localizes at the appositional surfaces of cells as they become confluent and is stably expressed in the junctional zones of confluent monolayers. This protein is expressed in situ on continuous endothelia of all blood vessels in all human tissues examined. Moreover, this protein, as determined by mAb immunocytochemistry, is not expressed by any other cell type. This protein may mediate endothelial-specific functions restricted to the intercellular domain. It may also serve as a unique cell surface marker for the identification and purification of human endothelial cells.

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