Model of PECAM-1 homophilic interactions that form at the endothelial cell-cell junction. Both IgD1 and IgD2 play important roles in creating, and then solidifying, homophilic PECAM-1/PECAM-1 interactions. Note the concentration-dependent interfaces that form between IgD1/D1, IgD1/D2, and IgD2/D2 in closely apposed PECAM-1 molecules. Inset, Human umbilical vein endothelial cells immunostained with mAb PECAM-1.3, with the proposed homophilic PECAM-1/PECAM-1 model superimposed to generate a schematic of the junctional structure that may exist at endothelial cell-cell borders. PECAM-1 molecules are not to scale.