CD31 (PECAM1) is a member of the immunoglobulin (Ig) superfamily that is expressed on the surface of circulating platelets, monocytes, neutrophils, and particular T-cell subsets. CD31 is also a major constituent of the endothelial cell intercellular junction, where up to an estimated 1 million molecules are concentrated. Because of this cellular expression pattern, CD31 is implicated in several functions, including transendothelial migration of leukocytes, angiogenesis, and integrin activation. Ig superfamily members are known to mediate cell adhesion, or antigen recognition, e.g. immunoglobulins, T-cell receptors, and MHC molecules. In addition, a subgroup comprising 30 members characterized by the presence of one or more immunoreceptor tyrosine-based inhibitory motifs (ITIMs) within their cytoplasmic domain has also been recognized. CD31, which has 6 ITIMs within its cytoplasmic domain, is a member of this subfamily.