To identify immune cells that had taken up tumor-derived GFP and thus interacted with the tumor (Fig. 1a), we implanted syngeneic mouse glioblastoma cells, GL261.BpalmGFP, or carrier medium in adult C57BL6.CCR2RFP/WT mice that express red fluorescent protein (RFP) under the CCR2 promoter in peripheral blood monocytes and monocyte-derived macrophages, but not in microglia [21]. This evidence concerns the gene CCR2 and glioblastoma.