CD81 and neoplasm: At present, it is well-established than a hypoxic microenvironment stimulates the secretion of large amounts of vascular endothelial growth factor (VEGF) which locally promotes angiogenesis and tumor growth [140], together with the loss of expression of several (stroma) adhesion molecules on tumor PC [20,146], such as CD56, CD138, CD81 and CD117 [21,144].