The GPCR CXCR6 (previously known as BONZO, STRL33 and TYMSTR) binds selectively to the free chemokine domain of CXCL16, which is derived from a membrane-bound precursor containing a CXC-containing chemokine domain, a glycosylated mucin-like domain, and a transmembrane domain (Wilbanks et al., 2001). CXCR6 is selectively expressed on Th1, Th2 and Tr1 T cell subsets, whereas CXCL16 is expressed on monocytes/macrophages and dendritic cells (Tabata et al., 2005). CXCR6 functions as a cofactor with CD4 for HIV entry and Env-mediated cell fusion (Liao et al., 1997). Binding of CXCL16 to CXCR6 promotes migration of activated lymphocytes to sites of inflammation in tissues such as liver and synovium (Nanki et al., 2005, Sato et al., 2005). Millipore’s cloned human CXCR6 -expressing cell line is made in the Chem-5 host, which supports high levels of recombinant CXCR6 expression on the cell surface and contains high levels of the promiscuous G protein to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for agonists and antagonists of CXCR6.