Junctional adhesion molecule A is a protein that in humans is encoded by the F11R gene. It has also been designated as CD321 (cluster of differentiation 321). Tight junctions represent one mode of cell-to-cell adhesion in epithelial or endothelial cell sheets, forming continuous seals around cells and serving as a physical barrier to prevent solutes and water from passing freely through the paracellular space. The protein encoded by this immunoglobulin superfamily gene member is an important regulator of tight junction assembly in epithelia. In addition, the encoded protein can act as a receptor for reovirus, a ligand for the integrin LFA1, involved in leukocyte transmigration, and a platelet receptor.