CX3CR1, (CX3C-chemokine receptor 1, fractalkine receptor or GPR13) is a receptor for the CX3C chemokine fractalkine. CX3CR1 is expressed in cytotoxic effector lymphocytes, including natural killer cells, cytotoxic T lymphocytes and macrophages. Soluble fractalkine causes migration of these cells, whereas the membrane-bound form captures and enhances the subsequent migration in response to secondary stimulation with other chemokines. Furthermore, stimulation through membrane-bound fractalkine activates natural killer cells, leading to increased cytotoxicity and interferon-gamma production. Fractalkine is involved in the pathogenesis of various clinical disease states or processes, such as atherosclerosis, glomerulonephritis, cardiac allograft rejection and rheumatoid arthritis. In addition, polymorphisms in CX3CR1, which reduce its binding activity to fractalkine, have been reported to increase the risk of HIV disease and to reduce the risk of coronary artery disease.