Clone 9C4 interferes with chemotaxis of T-cells by specifically blocking CXCL12/SDF-1 from binding to one of its receptors, CXCR7. CXCR7, also called GPR159 and RDC1, is a putative surface marker of memory B cells. It is also expressed on monocytes and dendritic cells during the final stages of maturation, as well as in fetal liver cells, some tumor cells and activated endothelial cells. CXCR7 is also a co-receptor for some HIV strains. While other published CXCR7 monoclonals, possibly due to secondary modifications on the antigen, react with either lymphocytes or CXCR7 expressed tumor cells, 9C4 reacts with. Clone 9C4 is therefore expected to be not only a useful marker to investigate the development of B cells and dendritic cells, but also a specific tool to dissect the chemotactic function of CXCL12 on T-cells and the role of CXCR7 in tumor growth.