CX3CL1 (fractalkine) is a member of the chemokine superfamily and functions as a human leukocyte chemoattractant protein (Bazan JF et al, 1997). Unlike other human chemokines, the chemokine domain of fractalkine has three amino acids between two conserved cysteines, referred to as the CX3C motif. This molecule can exist in two forms: either membrane-anchored or as a shed 95K glycoprotein. The soluble form has potent chemoattractant activity for T-cells and monocytes, and the membrane-bound protein, which is induced on activated primary endothelial cells, promotes strong adhesion of those leukocytes. The seven-transmembrane high-affinity receptor for fractalkine, termed CX3C1, mediates both the adhesive and migratory functions of fractalkine (Imai T et al, 1997). CX3CL1 is reported to signal via Gi (Brandt et al. 2002).