CD31, also known as PECAM-1, is a 130 kDa integral membrane glycoprotein found on the surface of endothelial cells, platelets and some hematopoietic cells. The antibody labels endothelial cells of arteries, arterioles, venules, veins, and non-sinusoidal capillaries in various tissues. CD31 is the most sensitive and specific endothelial cell marker. It is useful for detection of tumors with endothelial origin. In addition, CD31 has been used to identify vascular invasion of tumors, and assessment of angiogenesis which is a prognostic marker for many types of cancer.

CD31 is present on hematopoietic stem cells (HSCs), and its expression is used to determine the concentration of HSCs in research studies and for bone marrow transplantation. Anti-CD31 is very specific and sensitive for endothelial cells and does not stain non-vascular tumors, therefore CD31 staining is used to recognize the vascular origins of neoplasms.