The orphan receptor RDC1 is phylogenetically related to chemokine receptors, and its gene (CMKOR1, chemokine orphan receptor 1) occupies a chromosomal location near the genes for other CXC receptors. Several immune system cells, including T lymphocytes, mature dendritic cells and memory B cells, express RDC1 (Infantino et al., 2006). A subset of HIV and SIV strains use RDC1 as a coreceptor (Shimizu et al., 2000). The chemokine SDF-1/CXCL12, which binds to CXCR4, has been shown also to bind and activate RDC1 to promote chemotaxis of T lymphocytes; thus, CXCR7 has been proposed to be the new name for RDC1 (Balabanian et al., 2005). I-TAC, one of the chemokines thought to bind to CXCR3 only, has been reported to distinguish CXCR7 from CXCR4 in radioligand binding experiments (Burns et al. 2006). CXCR7 membrane preparations are crude membrane preparations made from our proprietary stable recombinant cell lines to ensure high-level of GPCR surface expression; thus, they are ideal HTS tools for screening of agonists and antagonists at CXCR7. The membrane preparations exhibit a Kd of 0.32 nM for [125I]-SDF-1a. With 0.25 nM [125I]-SDF-1a, 10mg/well CXCR7 Membrane Prep typically yields greater than 4-fold signal-to-background ratio.