Platelet endothelial cell adhesion molecule PECAM1 (also called CD31) is a 130 kDa type I transmembrane glycoprotein that is expressed on most cells of the hematopoietic lineage including platelets, monocytes, neutrophils and certain lymphocyte subsets (1). It has also been suggested that PECAM-1 contributes to endothelial cell barrier function.