In the context of metastasis, syndecan-1 (CD138) on 4T1 cancer cell membranes can bind to endothelial cell adhesion molecule-1 (CD31) on thrombocytes, endothelial cells, and leukocytes (e.g., monocytes and neutrophils) surrounding the brain vasculature, thereby promoting adhesion and facilitating migration across the blood–brain barrier [24,25,26]. The gene discussed is PECAM1; the disease is cancer.