CXCR4 and neoplasm: CXCL12 exerts also pro-angiogenic activity, recruiting CXCR4-positive, circulating bone marrow-derived cells (Petit et al., 2007) and promoting tumor vasculature recovery after irradiation, as a consequence of treatment-induced hypoxia and HIF-1α activation, which results in increased CXCL12 expression (Jin et al., 2006; Kioi et al., 2010).