Cluster of differentiation 31 (CD31), also known as platelet endothelial cell adhesion molecule (PECAM-1), plays a key role in removing aged neutrophils from the body. It is found on the surface of platelets, monocytes, neutrophils, and some types of T cells. CD31 is also expressed in certain tumors, including epithelioid hemangioendothelioma, epithelioid sarcoma-like hemangioendothelioma, other vascular tumors, histiocytic malignancies, and plasmacytomas. The encoded protein is a member of the immunoglobulin superfamily and is likely involved in leukocyte transmigration, angiogenesis, and integrin activation. In immunohistochemistry, CD31 is used primarily to demonstrate the presence of endothelial cells in histological tissue sections. This can help to evaluate the degree of tumour angiogenesis. CD31 immunohistochemistry can also be used to demonstrate both angiomas and angiosarcomas. It can also be demonstrated in small lymphocytic and lymphoblastic lymphomas.