Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a member of the immunoglobulin gene superfamily of cell adhesion molecules. It is highly expressed on the surface of the endothelium, making up a large portion of its intracellular junctions. CD31 is also present on the surface of hematopoietic cells and immune cells including platelets, monocytes, neutrophils, natural killer cells, megakaryocytes and some types of T-cell. As well as its role in cell-cell adhesion, CD31 functions as a signaling receptor, and is involved in important physiological events such as nitric oxide production, regulation of T-cell immunity and tolerance, leukocyte transendothelial migration and inflammation and angiogenesis. Soluble form of CD31 is present in human plasma, resulting from alternative splicing of the transmembrane domain.