CD31, a 120–140 kDa single-chain transmembrane glycoprotein, is present on virtually all monocytes, platelets, and granulocytes. CD31 is also expressed by lymphocyte subsets. During maturation of CD4+
T cells, the expression of CD31 changes: CD4+
recent thymic emigrants (RTEs) express CD31 in contrast to central naive T cells. CD31 is highly expressed on endothelial cells, especially at their juncture, and is also known EndoCAM.