CXCR4 and glioblastoma: Immunohistochemical studies showed that in GBM CXCR4 and CXCL12 expression does not co-localize with tumor proliferating cells (identified by MIB-1 expressing cells) but they are both mainly localized in hypoxic regions, characterized by necrosis (Rempel et al., 2000; Salmaggi et al., 2005b; Zagzag et al., 2008).