CXCL12 and neoplasm: Ionizing radiation has been demonstrated in vitro as well as in vivo to stimulate SDF-1/CXCR4 signaling in different human and mouse tumor entities either directly by S-nitrosylation and stabilization of HIF-1α (134) or indirectly via radiation-induced endothelial cell killing and resulting hypoxia (135) or HIF-1α-independent mechanisms (136).