In several co-culture experimental systems of breast cancer cells and endothelial cells, the highly-invasive breast cancer cells have previously been reported to break down the endothelial barriers through the reduction of cell–cell adhesion molecule expressions, such as platelet endothelial cell adhesion molecule-1 (PECAM-1) and vascular endothelial cadherin (VE-cadherin) [7]. Here, PECAM1 is linked to breast cancer.