CD31 (PECAM-1) is a surface glycoprotein expressed on the surface of endothelial cells, platelets, granulocytes, and monocytes that acts as an inhibitory receptor in circulating platelets and leukocytes as well as n endothelial adhesive stress response protein. It functions to promote and maintain endothelial cell junctional integrity and the vascular permeability barrier. It is also useful as an angiogenesis marker in cancer, and increased staining of vessels within lymph nodes is an indicator of potential lymph node metastases of invasive ductal carcinoma of the breast. This protein is expressed in endothelial cells within capillaries and vessels. References: Curr Opin Hamatol 2016 23(3):253; Popiela, 2008; Zaha, 2014