Platelet endothelial cell adhesion molecule (PECAM-1) also known as cluster of differentiation 31 (CD31) is a protein that in humans is encoded by the PECAM1 gene. CD31 is a cell-cell adhesion protein which interacts with other CD31 molecules through homophilic interactions or with non-CD31 molecules through heterophilic interactions. CD31 is normally found on endothelial cells, platelets, macrophages and Kupffer cells, granulocytes, lymphocytes (T cells, B cells, and NK cells), megakaryocytes, and osteoclasts. 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 tumor angiogenesis, which can imply a rapidly growing tumor. Malignant endothelial cells also commonly retain the antigen, so that CD31 immunohistochemistry can also be used to demonstrate both angiomas and angiosarcomas. It can also be demonstrated in small lymphocytic and lymphoblastic lymphomas, although more specific markers are available for these conditions.
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