Tumour stroma is formed by various infiltrating and locally differentiated cell populations, e.g., tumour-associated macrophages (TAMs: F4/80+), DCs (CD11c+), myeloid-derived suppressor cells (MDSCs: CD11b+ and granulocyte receptor [Gr]-1+), cancer-associated fibroblasts (CAFs: fibroblast markers [e.g., murine ER-TR7+] and α-smooth muscle actin [SMA]+), and mesenchymal stem cells (MSCs: platelet-derived growth factor-α [PDGFRα: CD140a]+ and stem cell antigen [Sca]-1+)10 along with tumour angiogenesis (Sca-1+ and CD31+)11 to fill gaps in tumour areas with extracellular matrix proteins12,13. The gene discussed is PECAM1; the disease is neoplasm.