CCR2 is a GPCR that is expressed on monocytes, dendritic cells, natural killer cells, basophils and neutrophils, and binds to MCP-1, 3 and 4, members of the MCP (monocyte chemoattractant protein) family of chemokines (Olson and Ley, 2002). Alternative splicing of the CCR2 gene results in two variants, CCR2A and CCR2B, that differ in their C-terminal tails (Wong and Charo, 1997). CCR2 plays a protective role in some conditions: mice lacking the gene encoding CCR2 develop age-related macular degeneration (Ambati et al., 2003), and in experimental models of rheumatoid arthritis, Ccr2-null mice develop a phenotype of greater severity than wild-type mice (Quinones et al., 2004). However, CCR2 appears to promote the pathogenesis of graft-versus-host disease and pulmonary and kidney fibrosis (Kitagawa et al., 2004; Rao et al., 2003). Cloned human CCR2-expressing cell line is made in the Chem-1 host, which supports high levels of recombinant CCR2 expression on the cell surface and contains high levels of the promiscuous G protein Gα15 to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for agonists, antagonists and modulators at CCR2.