CD31, also known as Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1), is a 130 kDa type I transmembrane glycoprotein that belongs to the immunoglobulin superfamily. It is widely expressed on the surface of various cells, including endothelial cells, circulating platelets, monocytes, neutrophils, and certain T cell subsets. CD31 plays a crucial role in cell-cell adhesion and signaling, particularly in immune cell transmigration and angiogenesis. It is involved in the clearance of senescent neutrophils and likely plays a role in leukocyte migration, vascular integrity, and the activation of integrins. Activation of CD31 has been linked to signaling events that can trigger both atherogenesis and anti-atherogenic endothelial NO production, suggesting a complex physiology dependent on various factors. CD31 is considered a potential biomarker for the efficacy of bevacizumab in ovarian cancer, as the impact of bevacizumab on progression-free survival and overall survival is most pronounced in patients with high CD31 expression. It is also used in immunohistochemistry to identify endothelial tissues, assess tumor angiogenesis, and diagnose vascular tumors.