Fractalkine (CX3CL1) is the unique member of the CX3C chemokine subfamily. In contrast to other chemokines, it exists in two forms, each mediating distinct biological actions. The membrane-anchored protein, which is primarily expressed on the inflamed endothelium, serves as an adhesion protein promoting the retention of monocytes and T cells in inflamed tissue. The soluble form resembles more a conventional chemokine and strongly induces chemotaxis. Both chemotaxis and adhesion are mediated by the G protein-coupled receptor CX3CR1. CX3CL1 has been thought to play an important role in inflammation, and indeed, accumulating evidence indicates that CX3CL1/CX3CR1 are involved in the pathogenesis of various inflammatory disorders such as glomerulonephritis, rheumatoid arthritis and systemic lupus erythematosus (SLE).