Fractalkine, also termed neurotactin, is a membrane-bound CX3C chemokine1. Rat CX3CR1, also named RBS11, was first cloned from rat brainstem, pituitary and/or spinal cord cDNA libraries2. A G-protein-coupled seven-transmembrane domain receptor, it was recently identified to serve as fractalkine receptor3. The numan equivalent receptor is known as V28, which has been shown to mediate both the adhesive and migratory functions of fractalkine. Fractalkine and CX3CR1 represent new types of leukocyte trafficking regulators.