CD31, also known as PECAM-1, is a member of adhension molecular that is expressed at high levels on early and mature endothelial cells, platelets, and most leukocyte subpopulations. The expression on endothelial cells is concentrated at junctions between adjacent cells. CD31 is also expressed on a major population of macrophage/dendritic cell precursors in the bone marrow. PECAM-1 is known to have various roles in vascular biology including angiogenesis, platelet function, and thrombosis. It is a mechanosensor of endothelial cell response to fluid shear stress and it is involved in the regulation of leukocyte migration through venular walls.