Description: CD31 (PECAM-1) is a transmembrane glycoprotein and is widely used as an endothelial cell marker. Anti-CD31 reacts with normal, benign, and malignant endothelial cells which make up blood vessel lining. The level of expression can help to determine the degree of tumor angiogenesis, a high level may imply a rapidly growing tumor and potentially a predictor of recurrence.