CXCR4 and glioblastoma: In both native U251MG and U87MG GBM cells, CXC receptor 4 (CXCR4) (ligand CXCL12), CXCR6 (ligand CXCL16), and CX3C receptor 1 (CX3CR1) (ligand CX3CL1) were not expressed (Figure 6C,D).