CD31, also known as platelet endothelial cell adhesion molecule-1 (PECAM-1), is a type I integral membrane glycoprotein and a member of the immunoglobulin superfamily of cell surface receptors. It is constitutively expressed on the surface of endothelial cells and concentrated at the junction between them. It is also weakly expressed on many peripheral lymphoid cells and platelets. CD31 interacts homotypically in cell adhesion assays. The monoclonal antibody 390 inhibits the aggregation of L cells transfected with a variant form of CD31.