PECAM1 and neoplasm: CD31 immunostaining of xenografts carried out using an antibody specific for murine CD31 (unable to recognize human CD31) showed a broad extensive vessels network supporting murine cells involvement into tumor-angiogenesis, besides no merge between GFP-positive and CD31-positive cells was found, rather GFP-positive cells localized next to endothelial cells (Figure 4D) as pericytes do, supporting endothelial vasculature.