CXCR4 and cancer: Moreover, exposure of CXCR4+CXCR7+ cancer cells to CXCL12 greatly enhances migration of human Burkitt’s lymphoma cells through a human HUVEC endothelial cell monolayer as in vitro model of transendothelial migration (Zabel et al., 2011) suggesting the potential efficacy of CXCR7 antagonists in blocking CXCL12-mediated metastatic spread of CXCR4+CXCR7+ tumor cells, in vivo.