CXCR4 and neuroblastoma: The differences in the involvement of CXCR4 and CXCR7 receptors between in vitro and in vivo NB cell invasion can be explained by the fact that in vivo invasion results from an environment-dependent multistep process, involving the ability of tumor cells to arrest in a specific organ, adhere to endothelial cells, extravasate, and establish an appropriate microenvironment for proliferation.